Semiconductor structure processing using multiple laser beam spots spaced on-axis with cross-axis offset

ABSTRACT

Methods and systems selectively irradiate structures on or within a semiconductor substrate using a plurality of laser beams. The structures are arranged in a row extending in a generally lengthwise direction. The method generates a first laser beam that propagates along a first laser beam axis that intersects the semiconductor substrate and a second laser beam that propagates along a second laser beam axis that intersects the semiconductor substrate. The method directs the first and second laser beams onto distinct first and second structures in the row. The second spot is offset from the first spot by some amount in a direction perpendicular to the lengthwise direction of the row. The method moves the first and second laser beam axes relative to the semiconductor substrate along the row substantially in unison in a direction substantially parallel to the lengthwise direction of the row.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Application No. 60/580,917, entitled “Multiple-BeamSemiconductor Link Processing,” filed Jun. 18, 2004, which isincorporated by reference herein in its entirety. Also incorporated byreference herein are the following commonly owned U.S. patentapplications filed contemporaneously with this application:

-   -   application Ser. No. 10/______ (attorney docket no.        50001/112.2), entitled “Semiconductor Structure Processing Using        Multiple Laterally Spaced Laser Beam Spots with On-Axis Offset”;    -   application Ser. No. 10/______ (attorney docket no.        50001/112.3), entitled “Semiconductor Structure Processing Using        Multiple Laterally Spaced Laser Beam Spots Delivering Multiple        Blows”;    -   application Ser. No. 10/______ (attorney docket no.        50001/112.4), entitled “Semiconductor Structure Processing Using        Multiple Laterally Spaced Laser Beam Spots with Joint Velocity        Profiling”;    -   application Ser. No. 10/______ (attorney docket no.        50001/112.5), entitled “Semiconductor Structure Processing Using        Multiple Laser Beam Spots Spaced On-Axis Delivered        Simultaneously”;    -   application Ser. No. 10/______ (attorney docket no.        50001/112.6), entitled “Semiconductor Structure Processing Using        Multiple Laser Beam Spots Spaced On-Axis to Increase Single-Blow        Throughput”;    -   application Ser. No. 10/______ (attorney docket no.        50001/112.7), entitled “Semiconductor Structure Processing Using        Multiple Laser Beam Spots Spaced On-Axis on Non-Adjacent        Structures”; and    -   application Ser. No. 10/______ (attorney docket no.        50001/112.9), entitled “Semiconductor Structure Processing Using        Multiple Laser Beam Spots Overlapping Lengthwise on a        Structure”.

TECHNICAL FIELD

This disclosure relates generally to manufacturing semiconductorintegrated circuits and more particularly to the use of laser beams toprocess structures on or within a semiconductor integrated circuit.

BACKGROUND

During their fabrication process, ICs (integrated circuits) often incurdefects for various reasons. For that reason, IC devices are usuallydesigned to contain redundant circuit elements, such as spare rows andcolumns of memory cells in semiconductor memory devices, e.g., a DRAM(dynamic random access memory), an SRAM (static random access memory),or an embedded memory. Such devices are also designed to includeparticular laser-severable links between electrical contacts of theredundant circuit elements. Such links can be removed, for example, todisconnect a defective memory cell and to substitute a replacementredundant cell. Similar techniques are also used to sever links in orderto program or configure logic products, such as gate arrays or ASICs(application-specific integrated circuits). After an IC has beenfabricated, its circuit elements are tested for defects, and thelocations of defects may be recorded in a database. Combined withpositional information regarding the layout of the IC and the locationof its circuit elements, a laser-based link processing system can beemployed to remove selected links so as to make the IC useful.

Laser-severable links are typically about 0.5-1 microns (μm) thick,about 0.5-1 μm wide, and about 8 μm in length. Circuit element in an IC,and thus links between those elements, are typically arranged in aregular geometric arrangement, such as in regular rows. In a typical rowof links, the center-to-center pitch between adjacent links is about 2-3μm. These dimensions are representative, and are declining astechnological advances allow for the fabrication of workpieces withsmaller features and the creation of laser processing systems withgreater accuracy and smaller focused laser beam spots. Although the mostprevalent link materials have been polysilicon and like compositions,memory manufacturers have more recently adopted a variety of moreconductive metallic link materials that may include, but are not limitedto, aluminum, copper, gold nickel, titanium, tungsten, platinum, as wellas other metals, metal alloys, metal nitrides such as titanium ortantalum nitride, metal silicides such as tungsten silicide, or othermetal-like materials.

Conventional laser-based semiconductor link processing systems focus asingle pulse of laser output having a pulse width of about 4 to 30nanoseconds (ns) at each link. The laser beam is incident upon the ICwith a footprint or spot size large enough to remove one and only onelink at a time. When a laser pulse impinges a polysilicon or metal linkpositioned above a silicon substrate and between component layers of apassivation layer stack including an overlying passivation layer, whichis typically 2000-10,000 angstrom (Å) thick, and an underlyingpassivation layer, the silicon substrate absorbs a relatively smallproportional quantity of infrared (IR) radiation and the passivationlayers (silicon dioxide or silicon nitride) are relatively transparentto IR radiation. Infrared (IR) laser wavelengths (e.g., 0.522 μm, 1.047μm, 1.064 μm, 1.321 μm, and 1.34 μm) have been employed for more than 20years to remove circuit links.

Present semiconductor link processing systems employ a single laserpulse focused into a small spot for link removal. Banks of links to beremoved are typically arranged on the wafer in a straight row, anillustrative one of which is shown in FIG. 1. The row need not beperfectly straight, although typically it is quite straight. The linksare processed by the system in a link run 120, which is also referred toas an on-the-fly (“OTF”) run. During a link run, the laser beam ispulsed as a stage positioner passes the row of links across the focusedlaser spot location. The stage typically moves along a single axis at atime and does not stop at each link position. Thus the link run is aprocessing pass down a row of links in a generally lengthwise direction(horizontally across the page as shown.) Moreover, the lengthwisedirection of the link run 120 need not be exactly perpendicular to thelengthwise direction of the individual links that constitute the row,although that is typically approximately true. Impingent upon selectedlinks in the link run 120 is a laser beam whose propagation path isalong an axis. The position at which that axis intersects the workpiececontinually advances along the link run 120 while pulsing the laser toselectively remove links. The laser is triggered to emit a pulse andsever a link when the wafer and optical components have a relativeposition such that the pulse energy will impinge upon the link. Some ofthe links are not irradiated and left as unprocessed links 140, whileothers are irradiated to become severed links 150.

FIG. 2 illustrates a typical link processing system that adjusts thespot position by moving a wafer 240 in an XY plane underneath astationary optics table 210. The optics table 210 supports a laser 220,a mirror 225, a focusing lens 230, and possibly other optical hardware.The wafer 240 is moved underneath in the XY plane by placing it on achuck 250 that is carried by a motion stage 260.

FIG. 3 depicts the processing of the wafer 240. A conventionalsequential link blowing process requires scanning the XY motion stage260 across the wafer 240 once for each link run. Repeatedly scanningback and forth across the wafer 240 results in complete waferprocessing. A machine typically scans back and forth processing allX-axis link runs 250 (shown with solid lines) before processing theY-axis link runs 260 (shown in dashed lines). This example is merelyillustrative. Other configurations of link runs and processingmodalities are possible. For example it is possible to process links bymoving the wafer, optics rail, or through beam deflection. In addition,link banks, and link runs may not be straight rows and may not beprocessed with continuous motion.

For this example, the primary system parameters that impact the timespent executing link runs, and thus throughput, are the laser pulserepetition frequency (PRF) and motion stage parameters such as stageacceleration, bandwidth, settling time, and the commanded stagetrajectory. The commanded stage trajectory consists of acceleration anddeceleration segments, constant velocity processing of link banks, and“gap profiling” or accelerating over large gaps between links to beprocessed in a link run. Most improvements to system throughput over thepast several years have primarily focused upon enhancing the stage andlaser parameters. Improvements in these areas will continue; however,practical limitations associated with these parameters make this adifficult way to achieve large throughput gains.

Increasing peak stage acceleration, for example, provides only a limitedthroughput improvement. Present motion stages are capable of moving awafer with a full field travel, greater than 300 mm (millimeters), with1 to 2 G accelerations, while maintaining a positional accuracy on theorder of 100 nm (nanometers). Increasing stage acceleration introducesadditional vibrations and generates heat, both of which can decreasesystem accuracy. Significantly increasing the stage acceleration andbandwidth, without diminishing the positional accuracy or increasing thesystem footprint, is a challenging and costly engineering endeavor, andthe benefits of that effort would only be moderate.

Increasing the laser PRF, and hence link run velocity, is alsoundesirable for a number of reasons. First, there are unfavorablechanges in the laser pulses that result from increasing the PRF. For agiven laser cavity, as the inter-pulse period decreases, the laser pulsewidth increases. This may decrease the processing efficiency on somelink structures. Higher laser PRFs are also associated with less energystability, which also decreases processing efficiency. Higher laser PRFscan also result in lower pulse power, although that is usually not aproblem when processing links that use a small spot size.

High laser PRFs are also undesirable when applied to semiconductorproducts that have a large link pitch. The combination of high PRF andlarge link pitch requires that a very high stage velocity be used forprocessing links. A high stage velocity requires more stage accelerationand deceleration and decreases the opportunity to take advantage of gapsof unprocessed links in a run. These effects diminish some of thethroughput improvements from the higher link run velocity. A high stagevelocity also requires a tighter timing tolerance when triggering thegeneration of laser pulses in order to maintain accuracy. Processing athigh stage velocities may also not be possible if these velocitiesexceed some system specification, such as the maximum stage or positionfeedback sensor velocity.

Continued shrinkage of the feature sizes on semiconductor wafers willresult in an increased number of links and link runs to process thesewafers, further increasing wafer processing time, while future systemthroughput improvements of significant magnitude are unlikely to occurthrough improvements in stage acceleration performance or laser PRF.

SUMMARY

According to one embodiment, a method selectively irradiates structures(e.g., electrically conductive links) on or within a semiconductorsubstrate using a plurality of laser pulsed beams. The structures arearranged in a row extending in a generally lengthwise direction. Themethod generates a first laser beam that propagates along a first laserbeam axis that intersects the semiconductor substrate and a second laserbeam that propagates along a second laser beam axis that intersects thesemiconductor substrate. The method directs the first and second laserbeams onto distinct first and second structures in the row. The secondspot is offset from the first spot by some amount in a directionperpendicular to the lengthwise direction of the row. The method movesthe first and second laser beam axes relative to the semiconductorsubstrate along the row substantially in unison in a directionsubstantially parallel to the lengthwise direction of the row.

According to another embodiment, a system selectively irradiatesstructures on or within a semiconductor substrate using a plurality oflaser beams. The structures are arranged in a row extending in agenerally lengthwise direction. The system comprises a laser source, afirst laser beam propagation path, a second laser beam propagation path,and a motion stage. The laser source produces at least a first laserbeam and a second laser beam. The first laser beam propagates toward thesemiconductor substrate along the first laser beam propagation path. Thefirst laser beam propagation path has a first laser beam axis thatintersects the semiconductor substrate at a first spot. The second laserbeam propagates toward the semiconductor substrate along the secondlaser beam propagation path. The second laser beam propagation path hasa second laser beam axis that intersects the semiconductor substrate ata second spot. The first spot and the second spot impinge upon distinctfirst and second structures in the row. The first and second spots areseparated by some distance in a direction perpendicular to thelengthwise direction of the row. The motion stage moves the first andsecond laser beam axes relative to the semiconductor substratesubstantially in unison in a direction substantially parallel to thelengthwise direction of the row.

As used herein: the term “on” means not just directly on but atop,above, over, or covering, in any way, partially or fully; the term“substantially” is a broadening term that means about or approximatelybut does not imply a high degree of closeness; and the term “adjacent”means next to or next in a series (e.g., the letter “F” is adjacent to“G” but not “H” in the alphabet) without implying physical contact.

Additional details concerning the construction and operation ofparticular embodiments are set forth in the following sections withreference to the below-listed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a row or bank of links being selectivelyirradiated with a laser spot scanning along the lengthwise direction ofthe bank.

FIG. 2 is a diagram of a link processing system.

FIG. 3 is an illustration of link runs on a semiconductor wafer.

FIG. 4 is a velocity profile plot of a single link run.

FIG. 5 is an illustration of various two-spot arrangements, according tovarious embodiments.

FIG. 6 is an illustration of two different cases of two link rows inrelation to one another.

FIG. 7 is an illustration of two laterally spaced laser spots processingthe two cases of FIG. 6, according to one embodiment.

FIG. 8 is an illustration of two-spot and three-spot examples of on-axisarrangements of laser spots, according to two embodiments.

FIG. 9 is an illustration of a two-spot on-axis arrangement of laserspots for processing a row in one pass, according to one embodiment.

FIG. 10 is an illustration of a two-spot on-axis arrangement of laserspots for processing a row in two passes, according to one embodiment.

FIGS. 11 and 12 are illustrations of two laterally spaced laser spotswith relative steering in the lateral direction, according to oneembodiment.

FIG. 13 is an illustration of two cases of a four-spot arrangement withboth on-axis and cross-axis spacings, according to one embodiment.

FIG. 14 is a set of plots of laser pulse power versus time, according toone embodiment.

FIG. 15 is a block diagram of a multiple-spot laser processing system,according to one embodiment.

FIG. 16 is a block diagram of a two-spot laser processing system,according to one embodiment.

FIGS. 17-24 are diagrams of various implementations of a two-spot laserprocessing system, according to various embodiments.

FIG. 25 is a diagram of a system for combining multiple laser beams,according to one embodiment.

FIG. 26 is a diagram of a system for generating multiple laser beams,according to one embodiment.

FIG. 27 is a diagram of a multiple-lens laser processing system,according to one embodiment.

FIG. 28 is a diagram of a two-spot laser processing system with errorcorrection capability, according to one embodiment.

FIG. 29 is a diagram of a two-spot laser processing system withindependent beam steering, according to one embodiment.

FIG. 30 is a diagram of a two-spot laser processing system with energycalibration capability, according to one embodiment.

FIG. 31 is a diagram of a two-spot laser processing system with positioncalibration capability, according to one embodiment.

FIG. 32 is an illustration of a calibration target and two laser spots,according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the above-listed drawings, this section describesparticular embodiments and their detailed construction and operation.The principles, methods, and systems disclosed below have generalapplicability for processing any structure on or within a semiconductorsubstrate using laser radiation for any purpose. While the examples andembodiments that follow are described in the context in which thosestructures are laser-severable links on or within an IC (e.g., memorydevice, logic device, optical or optoelectronic device including LEDs,and microwave or RF devices), other structures besides laser-severablelinks can be processed in the same or similar manner, and the teachingsset forth herein are equally applicable to the laser processing of othertypes of structures, such as electrical structures that becomeconductive as a result of laser radiation, other electrical structures,optical or electro-optical structures, and mechanical orelectro-mechanical structures (e.g., MEMS (micro electro-mechanicalstructures) or MOEMS (micro opto-electro-mechanical structures)). Thepurpose of the irradiation may be to sever, cleave, make, heat, alter,diffuse, anneal, or measure a structure or its material. For example,laser radiation can induce a state change in a structure's material,cause the migration of dopants, or alter magnetic properties—any ofwhich could be used to connect, disconnect, tune, modify, or repairelectrical circuitry or other structures.

As one skilled in the art will appreciate in light of this disclosure,certain embodiments are capable of achieving certain advantages over theknown prior art, including some or all of the following: (1) increasingthroughput, possibly by multiplicative factors, e.g., by a factor of 2,3, 4, etc.; (2) decreasing floor space required for link processingequipment in a fabrication facility; (3) decreasing the time elapsingbetween scanning alignment targets and completing link processing,thereby (a) allowing less time for thermal drift of the components andstructure of the semiconductor processing system, resulting in enhancedsystem accuracy, (b) enabling larger wafer processing fields, whichresults in longer link runs and an additional throughput improvement,and (c) permitting less frequent rescanning of alignment targets whenthermal shifts are detected or when the time elapsed since theirprevious scan becomes too large, thus further enhancing throughput byreducing the number of operations necessary for accurate linkprocessing; and (4) allowing beneficial relaxation of some presentsystem parameters, such as XY stage acceleration and laser pulserepetition frequency, while still processing wafers at a rate that isequivalent or faster than present link processing systems. As an exampleof the latter advantage, lowering the stage acceleration requirementscan reduce the thermal energy released into the system environment,reducing thermal shifts that occur during wafer processing; loweracceleration will also improve accuracy by reducing the excitation ofsystem resonances and vibrations, resulting in smoother, gentler, morestable system operation; motion stages can also be selected with a lowercost, preferential mechanical configuration, greater simplicity, and noneed for auxiliary cooling systems if a reduced acceleration isacceptable. As another example, a laser source with a lower PRF could beused for processing; lower PRF lasers have improved pulse propertiessuch as faster rise time, enhanced pulse stability, increased peak pulsepower, and shorter pulse width; lower PRF lasers may also be less costlyand may be operable with smaller power supplies that generate less heat.These and other advantages of various embodiments will be apparent uponreading the remainder of this section.

I. Analysis of Link Run Processing Time

Measurements from the repair of typical DRAM wafers show that the timeto execute link runs accounts for the majority of wafer processing time.Approximately 85% of total processing time may be spent executing linkruns, and the remaining 15% is spent performing overhead tasks, such asmoving the wafer to shift the cutting laser from the end of one link runto the start of the next link run, alignment, focusing, andcomputational overhead. Because the dominant component of linkprocessing time is typically spent executing link runs, significantreductions in wafer processing time can result from reducing the timespent executing link runs.

FIG. 4 illustrates a link run velocity profile 410 corresponding to theprocessing of a link run 420. As used herein, the term “velocityprofile” means velocity as a function of time or distance over a span oftime or an interval of distance. Link run execution consists of a numberof different operations. While processing a bank 430 of links with atight pitch spacing (e.g., the center-to-center distance betweenadjacent links in the same bank), the laser beam axis advances relativeto the wafer at a nearly constant velocity 440. Note that, although FIG.4 shows an example in which the constant velocity 440 is the same foreach link bank 430 in the link run 420, it is possible that differentlink banks 430 may have different constant velocities, such as when thepitch spacing differs from bank to bank in the same link run. When thereis a large gap 450 between subsequent links in a link run, the systemaccelerates to span the gap 450 in less time and then decelerates nearthe end of the gap to reach a nominal velocity once again. Thatacceleration and deceleration result in a gap profile 460 in the linkvelocity profile 410. At the beginning of a link run, the systemundergoes an initial acceleration 470 from a resting position followedby a period of settling 480. At the end of a link run, the systemundergoes a deceleration 490 back down to zero velocity. Thus, thetypical operations that the system performs during execution of a linkrun include ramping up the stage to constant velocity, settling,processing links at constant velocity, accelerating (gap profiling) overany large gaps, and ramping back down to zero velocity at the end of therun. FIG. 4 illustrates the effect of these operations on link runon-axis velocity. Note that while the link run 420 is depicted as astraight line through co-linear link banks, it is possible for the banksof links to not be in line. Link run 420 would then contain lateralposition commands as well.

Insight into system improvements for reducing link run execution timeare evident from the following simplified throughput prediction model.The model approximates the time required for link run execution. Themodel is not accurate for absolute time prediction because it does notfully model all system behavior, such as move profiling; however therelative impact of changing different processing parameters is correct.According to this model, the time required to process the links isT _(LR) =N _(LR)({overscore (D)} _(LR) /{overscore (V)} _(LR) +T_(accel) +T _(settle) +T _(decel) −{overscore (T)} _(gap savings)).  (1)In Equation (1), T_(LR) is the total link run execution time and N_(LR)is the total number of link runs. The terms in parenthesis can be lumpedinto three categories: (1) time spent to span all the link runs atconstant velocity, (2) time spent to accelerate, settle, and decelerateduring link runs, and (3) time saved by gap profiling.

The average time spent on a link run at constant velocity is describedby {overscore (D)}_(LR) the average link run distance and {overscore(V)}_(LR) the average link run velocity. This velocity is typically{overscore (V)}_(LR)=P_(link)F_(laser) where P_(link) is fundamentallink pitch spacing and F_(laser) is the laser PRF.

A rearrangement of the terms of Equation (1) that pertain to constantlink run velocity shows that the total time spent at constant velocityis T_(vconst)=N_(LR){overscore (D)}_(LR)/{overscore(V)}_(LR)=D_(total)/{overscore (V)}_(LR). This can be restated asfollows: The total time required to process the link runs at constantvelocity is the total link run distance D_(total) divided by the linkrun velocity.

For the simplified throughput model, the time required to accelerate tothe link run velocity or decelerate from link run velocity isT_(accel)=T_(decel)={overscore (V)}_(LR)/A_(stage) where A_(stage) isthe stage acceleration, and the additional time required to elapse atthe end of the acceleration phase before processing links is thesettling time, denoted T_(settle). In actual implementations, morecomplex acceleration and deceleration profiles, such as half-sine ortrapezoidal profiling, are employed.

The final term in Equation (1), {overscore (T)}_(gap savings), is ameasure of the average time saved on a link run by gap profiling. A gapprofiling operation involves accelerating, decelerating, and settling totravel between two links in less time than would be required at constantvelocity. This magnitude of this term is dependent upon the quantity andspacing of large gaps between links, the acceleration capabilities ofthe stage, the settle time, and the link run velocity. A greater timesavings results on products that have many large gaps in link runs and asmall link pitch, hence a lower link run velocity.

The relative size of the three terms gives further insight into theimportance of different system changes. The time spent accelerating anddecelerating at the start and end of link runs is approximately 1.5% ofthe time spent on link runs. The time saved with gap profiling isapproximately 50% of the time that would be required to traverse thelink runs at constant velocity. These numbers vary widely for differenttypes of wafers. Workpieces with few or no large gaps between links willnot receive any benefit from gap profiling. On the other hand, productswith sparse or random link layout receive greater benefit from gapprofiling.

II. Parallelism in General

Parallel link processing by producing, and perhaps independentlycontrolling, multiple laser spots on a wafer surface can dramaticallyimprove system throughput.

In one implementation, the use of two focused laser spots allows onephysical pass of the wafer under the lens to result in the processing oftwo rows of links. Equation (1) shows that when processing multiplelaterally spaced link runs simultaneously, the link run execution timeis divided by the number of spots. For a two spot system, the XY stageonly needs to traverse the wafer by N_(LR)/2 times. The total link rundistance that the stage must travel is divided by two, and the number ofacceleration, deceleration, and settling events at the beginning and endof each link run is also divided by two. Although gap profiling timesavings may be divided by a number that approaches two, depending uponlink layout, the net result is that a laser system with two laterallyspaced spots requires about half the link run execution time.

The throughput improvements that result from multiple spot systems aremuch larger than can be accomplished through improvements to motionstage performance and laser PRF in a single spot system. In addition,these throughput improvements occur without any of the undesirableconsequences of pressing the laser and motion stage to higherperformance.

Multiple spot processing may take many different forms, with laserpulses being delivered to links with a different lateral (cross-axis)spacing, different on-axis spacing, different on-axis and cross-axisspacing, or no difference in link spacing. Each of these differentconfigurations offers different throughput and processing advantages andare explained in greater detail next, with reference to FIG. 5.

FIG. 5 depicts links being processed with some of the possible spacingsof two laser spots. The two laser spots are denoted “A” and “B” in thefigure. In the laterally (or cross-axis) spaced arrangement, spot A ison a link in one bank 510, while spot B is offset on a correspondinglink in a different, typically parallel, bank 520. Because spots A and Bpreferably advance in unison horizontally across the link runs 510 and520, as depicted in FIG. 5, the two spots can be said to be displacedrelative to one another in the cross-axis direction, with respect to thedirection of spot motion. Although we say that spots A and B advancealong their respective link banks, that is a linguistic shorthand. Moreprecisely, a spot results from a laser beam when the laser beam is on.In the case of an intermittent laser beam, such as a pulsed laser beam,the resulting spot on the IC workpiece comes and goes as the laser beamturns on and off. However, the laser beam propagates along an axis ofpropagation, and that axis always exists whether the beam is on or not.Thus, to be precise, a laser beam axis moves along the link run. At anygiven time during a link run, the axis intersects the IC workpieceeither on a link or between two adjacent links. When a laser beam axisintersects a link that has been selected for removal, the laser beam isenergized to sever the link. When the laser axis is moving along a bankof regular spaced links (with the approximately uniform pitch), thelaser beam can be pulsed periodically at a rate equivalent to andsynchronized in phase with the axis's crossing of links. The laserpulses can be selectively passed or blocked to sever a given link orleave it intact.

While the spots A and B are illustrated as having a circular shape inFIG. 5 and others, they may have any arbitrary shape that a laser beamcan produce.

As already mentioned, an advantage of laterally spaced spots is thatwafer processing can be accomplished with fewer link runs, resulting inmuch greater throughput without any laser or motion stage enhancements.Thus, from the perspective of increasing throughput, this is a valuableform of parallelism. However, parallelism can take a variety of forms,which can offer various advantages.

In an on-axis arrangement, spots A and B are on different links in thesame link bank 530 and may be substantially aligned along the axis ofspot motion. Although spots A and B are directed on adjacent links inthe FIG. 5 illustration, that need not be the case; for example, spot Amay lead spot B by two or more links, or vice versa. Advantages ofon-axis spaced laser spots include the following: (1) link run velocitycan be increased to enhance throughput, because the spots can advancetwice as far between pulses; (2) multiple laser pulses can be deliveredto a link during on-the-fly processing without repeating a link run; and(3) laser pulses with different properties can be selectively applied toa link.

Hybrids of both cross-axis and on-axis spacing are also possible, asshown in two illustrative examples in FIG. 5. In one arrangement, spotsA and B may be offset along the lateral axis while remaining on the samerow or bank 540 of links. Advantages of that single-row on-axis andcross-axis hybrid arrangement include better dissipation of energy inthe area between the two spots, as they are separated by a somewhatgreater distance than in the case without any cross-axis offset. Inanother arrangement, spots A and B fall on different banks 550 and 560and are offset in the on-axis direction as well. As IC feature sizescontinue to shrink, an on-axis offset between laterally spaced spots onadjacent rows can also result in better laser energy dissipation in thevicinity of the two spots, especially when pulsed simultaneously. Notethat processing in the on-axis and cross-axis configuration is possiblewith nearby link banks that are staggered, as shown in the on-axis andcross-axis configuration of FIG. 5, or regularly arranged as in thelayout of the cross-axis (lateral) configuration case.

Furthermore, in an overlap configuration, as shown twice in FIG. 5,spots A and B can be partially or substantially fully overlapping on thesame link in the same link bank 570 (full overlap) or 580 (partialoverlap). Advantages of multiple overlapped laser spots are that (1)laser spots with different optical properties can be selectivelydelivered to a link and (2) combining laser pulses that arrive atslightly different times is a method for temporally shaping theeffective combined pulse profile.

The two laser spots A and B may be processed simultaneously orsequentially. Simultaneous processing can result, for example, bysplitting a single laser beam into multiple spots or triggering twolasers to emit at the same time. Simultaneous delivery means atsubstantially the same time so that the time delay between pulse A andpulse B, at a link run velocity V_(LR), does not result in drift of theblow positions to such an extent that the drift is a substantialfraction of the focused beam spot diameter. For example, with a link runvelocity of 200 mm/sec and a desirable spot shift of less than 10% of a2 micron focused spot size, pulses arriving within 1 μsec of one anotherwould be considered simultaneous. Minor differences in the lengths ofoptical beam paths would result in time delays between pulses that aremuch less than this value, typically less than about 10 nsec.

Sequential spots, either generated from a single laser pulse that issplit and has long optical delay paths or generated from multiple laserpulses with a dwell between triggers, impinge upon the links withgreater time separation. Sequential spots may be utilized with multiplebeam paths by adjusting the relative positions of the focused laserspots on the target semiconductor wafer such that the focused spots areproperly positioned when pulse generation is triggered.

For multi-spot processing, the triggering of lasers to generate a pulsemay be based purely upon timing signals, or may be based upon actual,measured, estimated, or commanded positions of a spot, the workpiece, orthe workpiece relative to the spot. Pulse generation may also betriggered based upon average positions or estimated positions ofmultiple spots relative to multiple targets.

The subsequent sections describe various aspects of the various forms ofparallelism illustrated in FIG. 5.

III. Laterally Spaced Spots

Creating two or more focused laser spots that impinge upon adjacentlaterally (Cross-Axis) spaced banks of links is one configuration forimproving system throughput. By processing two or more banks of linkssimultaneously, the effective number of link runs and the distance theXY motion stage must travel during wafer processing is reduced by thenumber of laterally focused laser spots. For example, a wafer previouslyrequiring 1000 link runs with a single spot could be processed with only500 duplex link runs, where each duplex link run results in theprocessing of two laterally spaced banks of links. Cutting the number oflink runs in half results in a similar reduction in the time required toprocess the wafer. More generally, the use of N laterally spaced spotsresults in an throughput improvement on the order of N.

Laterally spaced spots typically improve throughput more than other spotconfigurations. Furthermore, the improvement in throughput poses no newdemands on the XY motion stage 260 or laser pulse rate requirementsbecause laterally spaced spots on separate rows can process thoseseparate rows simultaneously with the same velocity 440 as a single-spotsystem. However, when processing multiple rows during one link run itbecomes more important to perform the run at a velocity that iscompatible with all rows being processed. A compatible velocity profilefor N distinct rows to be processed jointly is a velocity profile thatis feasible, practicable, suitable, or appropriate for all of the Nparallel processes. Issues of velocity compatibility typically manifestthemselves in three primary forms. First, the constant velocity 440 forprocessing multiple parallel rows should be compatible with all of theprocessed rows. That can be ensured by utilizing a joint constantvelocity that is the minimum of the constant velocities 440 for theindividual link runs. In the typical case in which each row of links hasthe same pitch spacing, the constant velocities for each run is thesame; thus, ensuring that the joint constant velocity 440 is compatibleincurs no performance penalty. Second, gap profiling should becompatible with all of the processed rows. That can be ensured byemploying a gap profile 460 only where all processed rows have aligninggaps. Third, in cases in which the constant velocity 440 may differ fordifferent link banks 430 within the same link run 420, a simplificationresults from restricting link processing to constant velocity link runs,rather than allowing link run velocity to vary from one link bank 430 toanother. In general, a joint velocity profile for simultaneouslyprocessing multiple rows should be computed with all of the linkcoordinates of the parallel link runs taken into consideration forminimum pitch, appropriate areas for gap profiling, ramp-up andramp-down locations, and specific individual link run velocity profiles.Furthermore, where different laser sources have different propertiesaffecting the achievable velocity, such as PRF, those factors shouldalso be taken into account.

According to one embodiment, a joint velocity profile is computed byfirst computing the individual velocity profiles for each constituentrow in the run, and constructing a joint velocity profile that does notexceed the smallest maximum velocity value of any of the individualprofiles at each point along the profile. For example, if a first rowmust be processed at 125 mm/s or less in a segment, and a second rowmust be processed at 100 mm/s or less in the same segment, then thejoint velocity profile should be at 100 mm/s or less in that segment.

According to another embodiment, a joint velocity profile is determinedby computing the velocity profile for a single set of master linkcoordinates. The set of master link coordinates is generated from someor all of the link coordinates in the N rows being processed inparallel. The offset from each master link coordinate to a linkcoordinate in some or all of the N rows being processed jointly isdetermined, as well as information about whether each of the N pulseswill be blocked or transmitted to a target coordinate. The offset fromthe set of master link coordinates to one of the N rows being processedmay be zero. One way to process the parallel link runs is to useinformation derived from the offset coordinates to command motion of thebeam steering mechanisms, information derived from the set of masterlink coordinates to facilitate generation of laser pulses, andinformation about transmitting or blocking each of the N pulses todirect the switches that transmit or block pulses.

Software in the form of executable instruction code is a preferredmethod of computing a joint velocity profile.

To maximize joint velocity, the constituent link runs should be asspatially similar as possible. In other words, the same or similar pitchspacing should be utilized in corresponding banks of differentconstituent runs, and banks and gaps should be aligned when possible. Inthis way, intelligent layout of the links on the IC can facilitatelaterally spaced, multi-spot link processing that maximizes thethroughput advantage that such link processing can offer.

Throughput improvements involving laterally spaced spots do not requirean increase in stage velocity, as opposed to on-axis spaced spots which,for example, may require doubling the stage velocity of a single beamsystem. For this reason laterally spaced spots, which may also includean on-axis offset, are a preferred method of increasing throughput insystems that use high PRF lasers, such as those exceeding 30 kHz. As anexample, the basic link run velocity using a 40 kHz PRF laser and a 3 μmfuse pitch is 120 mm/s. Processing multiple link runs with laterallyspaced spots would use the same link run velocity. However, a two-spotsystem with on-axis spaced spots would preferably employ a 240 mm/s linkrun velocity, which exceeds the stage velocity limit of presentsemiconductor processing systems.

Laterally spaced spots are also a natural choice for use on many presentsemiconductor link layouts. A number of semiconductor devicemanufacturers produce products containing parallel link banks separatedby a distance of less than 200 μm. Center-to-center separation distancesof 10 μm or less are common. These types of link layouts presentlyresult because the width and separation distance of traces withinsemiconductors are typically smaller than the width and pitch spacing oflaser severable semiconductor links. Offset or staggered fuse designsresult from trying to pack the larger semiconductor links into a shorton-axis distance, as shown in FIG. 6. In some of these designs, thelinks have a purely lateral translation (shown on the left). In otherdesigns there is also an on-axis offset (shown on the right). Thebenefits of multiple-spot link processing with a pure on-axis spacingalso apply to processing with both an on-axis and a cross-axis spacing.

While the layout of many semiconductor designs is presently compatiblewith simultaneous processing of laterally spaced link banks, designersare trying to shrink the dimensions of laser severable fuses toeliminate the offset and staggered link configurations of FIG. 6 bylaying out fuses in a single row. This would enhance throughput onpresent single-spot systems.

To take greater advantage of multi-beam link processing, IC designerscan deliberately design semiconductor link layouts to be compatible withmulti-beam link processing. Creating products with a link layoutstargeted towards multi-beam processing, and in particular multi-beamprocessing with laterally spaced spots, may result in a dramaticincrease in throughput when processed on multi-beam systems. Desirablelink layouts for use with laterally spaced spots include link banks witha nearby center-to-center spacing, typically 10 μm or less, but possibly1 mm or more. It is also desirable to maximize system performance bylaying out the majority of links and link banks such that they can beprocessed with laterally spaced spots.

In one embodiment, a single laser pulse is split, with half the energydelivered to spot A and half to spot B. The use of optical switches canindependently select whether the pulses will be delivered to A or B,allowing the desired links to be properly severed.

FIG. 7 shows how a duplex link run might progress. A pair of laser spotsA₁ and B₁ correspond to a first laser pulse that has been split andapplied to two links. The next pulse emitted from the laser wouldimpinge upon the next two links as focused spots A₂ and B₂. Opticalswitches can select which pulses reach their target links. In the twoexamples depicted, pulses A₃, A₄, A₆, A₇, B₂, B₄, and B₈ reach andremove their target links. The other pulses are blocked, so they do notreach or alter any links.

In some cases, it is desirable to process links twice. Making two ormore passes along the link run to blow the links two or more times canbe easily accomplished with a multiple-spot link processing system.Because of the parallelism inherent to a multiple-spot link processingsystem, this can be accomplished significantly faster than with asingle-spot link processing system. Subsequent passes along a given bankof links may include a lateral shift of the focused laser spots so thatthe links are processed with different laser spots on each pass acrossthe wafer. For example, a first pass along a link run may selectivelyhit the links with spot A, and a second pass along the link run may hitthose same spots again with spot B while spot A advances along a newlink run.

It may also be useful to process wafers using multiple laterally spacedspots having different optical properties. Different optical propertiescan be achieved by inserting additional optical elements for changingpolarization, spot spatial distribution, spot size, wavelength, pulseenergy, or other optical property. Different optical properties can alsobe achieved by using different laser sources.

Since semiconductor wafers typically only contain one link design, theuse of spots with different optical properties would most likely beapplied in a double blow scenario. The first blow would partially removethe link and the second blow would clean out the blown link.Alternatively, the first blow may be passed or blocked and the secondblow would be passed or blocked, resulting in the application of one orthe other spot to the link. This is desirable in situations wheredifferent link properties or orientations make it preferable to processwith different laser spots. For example, if it is desired to processlinks with a polarized spot where the polarization direction correspondsto one axis of the link, spots A and B can be configured with differentpolarizations and applied to links with different orientations on theworkpiece. Application of spots with different optical properties willbe further detailed in subsection VI of this document.

While it is possible to use fixed optics in order to generate multiplelaterally spaced spots with a fixed offset, it is preferable to be ableto reconfigure the spot locations. Because most typical semiconductorproducts contain link layouts requiring link runs in both the X and theY axes, it is desirable to be able to reconfigure the spot translator sothat a lateral spot spacing can be created for either link rundirection. It is also desirable to be able to configure and adjust thespot spacing to match different link layouts.

IV. On-Axis Spaced Spots

Multiple spots distributed with an on-axis spacing, adjusted fore andaft along the axis of a link run, offer throughput and multiple-blowadvantages. In terms of throughput, this orientation can effectivelyincrease the laser PRF and link run velocity {overscore(V)}_(LR)=P_(link)F_(laser)N_(spot) by the number of spots N_(spot). Inthe case of two on-axis spaced spots, for example, the link run velocitydoubles. However, there may be less time savings through gap profilingdue to the increased base link run velocity.

The velocity profile of each link run should in most cases be computedwith all of multiple blow coordinates taken into consideration for,e.g., minimum pitch, appropriate areas for gap profiling, ramp-up andramp-down locations, and specific link bank velocity profiles.Computation of link run velocity profiles is known to those in the art.

FIG. 8 depicts a bank of links being processed by two- and three-spotimplementations of multiple on-axis laser spots. In one implementation,spots A and B—or A, B, and C—can originate from a single laser pulse andarrive at the work surface at substantially the same time. Opticalswitches controlled by the link processing system allow some pulses toreach the surface and sever links and other pulses to be blocked. In thetwo-spot case (top) shown in FIG. 8, spots A₁ and B₁ are processedapproximately simultaneously, then A₂ and B₂, then A₃ and B₃, etc. Inthe three-spot case (bottom) the three laser spots together advance inincrements of three along the link run (A₁, B₁ and C₁; then A₂, B₂ andC₂; then A₃, B₃ and C₃, etc.)

Implementing a fixed displacement or a steering mechanism to translatelaser spots by a distance greater than a single link pitch can preventsimultaneous processing of adjacent links. It is undesirable tosimultaneously process two adjacent links because of the increased pulseenergy that must be absorbed by the workpiece in the area between theadjacent links. This increased pulse energy can cause damage that wouldnot occur if the two adjacent links were not processed simultaneously.In other words, when the distance between spots is a multiple two orgreater times the pitch spacing, non-adjacent spots can be processed atthe same time with less likelihood of causing damage to the workpiece.

When the spacing between spots is an odd multiple of the link pitch (sothat laser spots fall upon, e.g., links 1 and 4, links 1 and 6, or links1 and 8, etc.), on-axis parallelism can be utilized advantageouslybecause one spot can process the “even” links in the link run whileanother spot processes the “odd” links. Such a spacing can also bedescribed as spacing the spots such that the number of links between thespots (not counting the endpoints) is an even number. In this case, linkrun velocity can be doubled while preventing the increased energy thatcould result from adjacent laser spots. FIG. 9 illustrates on-axisspacing with an increment of three link pitches (two links between thespots). This technique can be generalized to more than two spots whereall combinations of spots have an on-axis separation greater than thelink pitch.

A second way to process wafers with on-axis spots spaced by more thanone link pitch is to make two passes across a bank of links. An exampleof this is illustrated in FIG. 10. A first pass can selectively blowevery other link with two laser spots with a spacing of twice theminimum pitch, for example. The second pass can selectively blow theinterspersed links that were skipped on the first pass. Because the linkrun velocity is four times the velocity of the same laser applied to asingle spot, two passes may be completed and still improve systemthroughput. In FIG. 10, on the first pass pulses A₁, A₂, and B₂ reachedand removed link structures. The other pulses were blocked, but couldhave been applied to links. Even though different link run directionsare depicted in FIG. 10, the passes could have been completed with linkruns in the same direction.

Multiple spots with an on-axis spacing can also be used to efficientlydeliver multiple blows to a link. For example, some semiconductormanufacturers prefer to provide one pulse to sever the link followed bya second pulse to clean up the area and any residual link material. An,arrangement of multiple spots with an on-axis spacing is an effectiveway to deliver multiple pulses without making a second link run, aswould happen in a single-spot system.

An alternative way to deliver multiple blows to a link is to makemultiple passes down each link run. Because the link run velocity isgreater in a multiple-spot system than a single spot system, amultiple-spot system is preferable for this type of operation.

These techniques and hybrid combinations of these techniques can be usedto provide two or more blows to each link, and each blow may have thesame pulse property or different pulse properties.

Different optical properties can be achieved by inserting additionaloptical elements for changing polarization, spot spatial distribution,spot size, wavelength, pulse energy, or other optical property.Different optical properties, such as, for example, pulse width, canalso be achieved by using different laser sources. This can beparticularly useful for applying multiple blows to a link. One scenariowould be to apply a first pulse to a link, say at a higher energy levelto blow the link, and then following with a second pulse of lower energyto clean out any residual material. Application of spots with differentoptical properties will be further detailed in subsection VI below.

V. Combinations of Lateral and On-Axis Spacing

Different methodologies for spacing multiple laser spots with on-axisand cross-axis offsets offer different processing advantages. Thedual-row configuration shown on link banks 550 and 560 in FIG. 5 offersthe same advantages (e.g., enhanced throughput) as spots with across-axis spacing described above. Staggered links are not required forthis on-axis and cross-axis spacing technique. It can also be applied tothe regularly spaced links of the first example in FIG. 6. Processing asingle row at a time with an on-axis and cross-axis offsets of thefocused laser spots (e.g., as shown on link run 540 of FIG. 5) lowersthe laser fluence that reaches the silicon between the two link blows.The increased distance between the two spots when processing in thisconfiguration reduces the overlapped laser fluence to a level lower thanwould be achieved with a pure on-axis spacing.

Multi-beam link processing machines may also adjust the relative spacingof the laser spots during a link run for additional advantage. This maybe done for calibration or compensation purposes, for example therelative positioning of two or more links to be simultaneously processedmay not be uniform throughout the link run. Scale factors, rotations,and positional arrangements of the links may vary throughput the linkrun. Spot adjustment may also be done to compensate for detected errors.While adjustment of the beam spacing while processing banks of links ispossible, the beam spacing can be most easily adjusted during a gapbetween banks of links in a link run. A few examples of shifting spotsin both the on-axis and cross-axis while traversing a gap between banksof links are depicted in FIG. 11. An unlimited number of usefulreconfigurations of beam spacing is easily envisioned in light of theteachings herein.

Shifting from processing using cross-axis spaced spots to on-axis spacedspots, possibly combined with an increase or alteration in the link runvelocity, results in enhanced throughput in sections of the wafer wherelink banks have intermittent parallelism, as shown in the bottom portionof FIG. 11. Shifting from processing on-axis spaced spots to processingcross-axis spaced spots is also depicted in FIG. 11. Readily changingfrom any multiple-spot processing mode to any other multiple-spotprocessing mode is feasible and can be advantageous. A suitable jointvelocity profile can be computed that accommodates processing the linkrun in these different processing modes.

FIG. 12 depicts another way in which the relative spacing of two spotscould be adjusted to process links. In FIG. 12, spots A and B are twospots processed at the same or nearly the same time, and the lines andarrows in the figure indicate their relative positioning and adjustmentthroughout the link run. A fast actuator, such as an AOM (acousto-opticmodulator), can steer the beam(s) and thereby adjust the relative beamspacing before a link run or between subsequent links in a link run.

Although much of the above discussion has focused upon the creation oftwo-spot laser link processing systems, the principles and ideas may befurther extended to systems that use three, four, or more focused laserspots. Such multiple spots may be configured with lateral spacings,on-axis spacings, and also both on-axis and cross-axis spacings.

Processing links using an arrangement of cross-axis spaced and on-axisspaced pulses offers many of the advantages of both on-axis andcross-axis spaced pulses. For example, the number of link runs can bereduced, dramatically improving throughput, and the link runs may beprocessed at much greater velocity. In FIG. 13, pulses A, B, C, and Dall arrive substantially simultaneously at the links. As shown in thefirst example in FIG. 13 (shown on the left) below, the spots are notnecessarily arranged in a grid. Configurations can be applied thatelegantly match different device link layouts.

Processing numerous links in parallel with various on-axis and/orcross-axis focused spot configurations may require the consideration ofmore information in the generation of the link run trajectory andvelocity profile. All link blow coordinates, spot locations, and largegaps between link banks must be considered. This increases thecomplexity of the computation of the link run velocity, gap profilingsegments, and ramp-up and ramp-down distances.

VI. Multiple Beams on the Same Spot or Link

Another category of multiple laser spots arises when the spots are alldirected to overlap at a single target structure. Two advantages ofprocessing semiconductor link structures with overlapped spots are that(1) spots with different optical properties can be selectively chosenfor link processing and (2) small time delays can be used for temporalpulse shaping or spatial spot shaping. Furthermore, using spots thatonly partially overlap on the same link can enhance the reliability oflink severing without incurring a throughput penalty.

When starting with a single laser pulse, spots of different opticalproperties can be applied to a link by using additional optical elementsto alter the properties of the multiple beam paths. Alternatively,different laser heads can provide laser spots of different opticalproperties that are subsequently combined to overlap. It should also benoted that adjustable steering mirrors or beam deflectors are notrequired to implement spots that overlap at the work surface. Fixedoptical elements can be used.

When the multiple laser spots can be independently switched to reach ornot reach a target link, the link can then be processed with any or allof the beams. As an example, a system with two optical paths could haveone path with polarization aligned with the X-axis and the other pathwith a polarization aligned with the Y-axis. If it is desired to processa link with X-axis polarization, then the optical switch controlling theX-axis polarized beam allows the laser pulse to pass and the opticalswitch governing the transmissibility of the Y-axis beam is set to ablocking state. Alternatively, the link could be processed with a Y-axispolarization by passing just the Y-axis polarized spot. And, if desired,both optical switches could be opened to apply light of bothpolarizations to the link. This could be done to apply laser pulses thatdo not have a preferential polarization or have greater pulse energy tosome links.

If it is desired to only process a link with one of the multiple spotsresulting from independently switched, shuttered, or blocked laserbeams, then processing may occur without overlapping the spots. In thisspecial case, the multiple spots do not necessarily impinge upon thesame spot or link. It is advantageous to not require the overlap becausethe precision with which the laser beam paths are aligned may berelaxed. In the case where the multiple spots do not impinge on the samespot or link, links are processed by using the positioning mechanism foradjusting position of the workpiece relative to the focused laser spotssuch that the desired spot impinges upon the target link. For example,it may be desirable to process an X-axis link run with one of multiplespots and a Y-axis link run with another of multiple spots. Afterperforming a calibration procedure to determine how to target thedesired links and link runs with each desired spot, the link runs may besuccessfully executed with their respective spots.

Many different optical properties can be altered by inserting additionaloptics into the beam paths of a multi-beam system. Inserted optics maybe: (1) polarization altering elements to create or change thepolarization state of an optical pulse; (2) attenuators to change thepulse energies; (3) beam-shaping optics to change the spatialdistribution of the pulse (e.g., elliptical, gaussian, top-hat, ordoughnut (a spot with less energy in the center) profiled spots); (4)frequency-multiplying optics to change the wavelength of a pulse (ordifferent source lasers providing multiple pulses of differentwavelengths); (5) beam expanders to create different focused spot sizesat the link; and (6) lenses and relay optics. Other optical propertiesthat can differ between the multiple beam paths will be apparent tothose skilled in the art, as will be the appropriate optics to createthose optical properties.

A second use of multiple spots delivering energy to one link is temporalpulse shaping. Temporal pulse shaping can be accomplished by taking alaser pulse, splitting it into multiple beams that include time delayelements, and recombining the beams at a link structure. Through thistechnique, a pulse that has a fast rise time but a short duration can beeffectively stretched to have a fast rise time and a long pulseduration. Delayed pulses may be attenuated or be created with beamsplitters of varying ratios to offer additional flexibility in shapingthe pulse amplitudes.

FIG. 14 depicts a process of pulse shaping by combining delayed pulses.The top graph depicts a single laser pulse 610. The second graph depictsthis same pulse 610 plus a second pulse 620 of lower amplitude that hasbeen delayed by about 8 ns. The rise time is fast, and the effectivepulse duration is longer. The third graph depicts the primary pulse 610with the second pulse 620 and a third pulse 630, each of lower amplitudeand with increasing delays. The resultant additive waveform creates along pulse width of about 20 ns with the fast rise time of the originalpulse. This pulse shape is desirable for processing some semiconductorlink structures.

Delay can be added by introducing additional distance in an opticalpath. A delay of about 1 ns results from each foot of additional pathlength. Delay may result from, for example, simple beam routing,reflecting beams back and forth between mirrors, or dumping the beaminto a length of fiber-optic cable.

Pulse shaping can also be achieved by combining laser pulses ofdifferent properties from two different laser sources. For example ashort pulse with a quick rise time can be combined with a longer pulseof slow rise time to create a fast-rise, long-duration pulse.

Referring back to the partial overlap configuration 580 in FIG. 5, thetwo beam spots A and B partially overlap on the same link. In onepreferred embodiment, the two spots are offset along the lengthwisedirection of the link. That configuration can result in more reliablelink severing, especially when the spot size is small, as a single smallspot may not reliably sever a link. Rather than taking two passes with asingle laser spot to blow the links a second time in order to ensureseverance, the partial overlap configuration 580 can achieve the samereliability of severance without incurring a 50% throughput penalty.

As can be seen in FIG. 5, two laser beams propagate along respectivedistinct beam axes that intersect the same link at different locationsoffset from one another along the length of the link. Furthermore, thelasers subtend respective spots A and B on the surface of the link. Thespots A and B are non-concentric and define an overlapping region formedby the intersection of both spots and a total region formed by the unionof both spots. For example, the area of the overlapping region may be50% of the area of the total region. The energy levels of the two laserbeams is preferably set to ensure complete depthwise severance of thelink across its entire width in at least some portion of the totalregion (most likely the overlapping region). The energy ratio of the twobeams may be 1:1, but need not be so. Although the two laser beams mayimpinge upon the link at different times, any time delay is preferablysufficiently small (e.g., less than about 300 ns) to permit on-the-flylink severance as the laser beam axes scan along the bank of links. Notethat the case of two partially overlapping spots can be generalized tothree or more spots along the length of the link in a partiallyoverlapping pattern.

Utilizing multiple spots in conjunction with multiple beams delivered toeach spot is also a useful technique. For example, there could be threelaterally spaced focused spots with two different beam paths beingdelivered to each spot. This may be done to combine the throughputbenefits of multiple spots that do not overlap with the pulse creationand selection advantages of multiple spots focused upon a singlelocation. Thus three spots could each use multiple beams for temporalpulse shaping, spatial pulse shaping, or having the ability to selectbeams with different polarization states.

VII. Implementations

Parallelism can be accomplished using separate optical paths that aredelivered to target links through either a single focusing lens ormultiple focusing lenses. The separate optical paths may originate froma single laser or from multiple lasers. One implementation ofparallelism results from one laser head and one focus lens, as the blockdiagram in FIG. 15 shows. FIG. 15 depicts the basic functionality of anN-spot link processing system 700 with one laser 720 and one focus lens730. The laser beam output from the laser 720 is directed to a beamsplitter 745, which splits the laser beam into N beams, labeled beam 1through beam N. Each beam from the splitter 745 passes through a switch750, which can selectively pass or block the beam. The output of theswitch 750 goes to a beam steering mechanism 760, which may be fixed oradjustable. The beam steering mechanisms 760 direct the individual beamsto the focus lens 730, which focuses the beams onto N-spots on thesemiconductor device being processed (not shown). Although the beamsteering mechanisms 760 are preferably dynamically adjustable, they maybe fixed.

A special case of the N-spot system occurs when N=2. A block diagram ofa two-spot laser processing system 800 is depicted in FIG. 16. Thetwo-spot system 800 is like the N-spot system 700 but may containadditional optical elements 755, which are optional. The components ofthe N-spot system 700 and the two-spot system 800 may be implementedwith bulk optics, integrated optics, or fiber optics, for example. Theorder of some of the elements in the block diagrams may be rearranged(e.g., the switch 750 may be located after the additional opticalelements 755 in the beam paths).

FIG. 17 is a more detailed depiction of the principal components of oneform of the two-spot laser processing system 800. With reference to FIG.17, the two-spot system 800 operates as follows: The laser 720 istriggered to emit a pulse of light that is delivered to the beamsplitter 745. The beam splitter 745 divides the pulse into two separatepulses that will independently be delivered to the work surface 740. Onelaser pulse, shown as the solid line, travels to the work surface 740through a first, fixed optical path. The second laser pulse, shown as adotted line, travels to the work surface 740 through a second, fixedoptical path. Two switches 750, such as AOMs, are included so thateither pulse may be passed to the work surface 740 or blocked. Eachpulse is reflected off a mirror 762 and directed toward a beam combiner765. The pulse are recombined in the beam combiner 765, reflected off afinal mirror 725, and focused through a single focus lens 730 down tothe work surface 740, where they impinge upon semiconductor links.Additional optic elements 755 may also be included in the beam path tochange the optical properties of the pulses.

FIG. 17 also depicts components of the system 800 that control therelative movement of the workpiece 740 and the laser spots, thetriggering of the laser 720, and control of the switches 750, accordingto one illustrative control architecture. In particular, the workpiece740 is mounted to a motion stage 660 that moves the workpiece 740 in anXY plane (the laser beams being incident upon the workpiece in the Zdirection). One or more position sensors 680 sense where the workpiece740 is relative to one or both of the laser beam spots and reports thatposition data to a controller 690. The controller 690 also accesses atarget map 695, which contains data indicating target positions on theworkpiece 740 that should be irradiated (e.g., to sever a link at thatposition). The target map 695 is typically generated, for example, froma testing process that determines which circuit elements in theworkpiece 740 are defective or otherwise require irradiation, as well asfrom layout data and possibly alignment data. The controller 690choreographs the pulsing of the laser 720, the shuttering of theswitches 750, and the moving of the motion stage 660 so that the laserbeam spots traverses over each target and emit laser pulses that reachesthe workpiece 740 at the targets. The two spots of this basicimplementation may be applied to the work surface 740 with any desiredrelative XY spacing within the range of motion limits of the XY motionstage 660. The controller 690 preferably controls the system 800 basedon position data, as that approach provides very accurate placement oflink blows. U.S. Pat. No. 6,172,325, assigned to the assignee of thepresent invention and incorporated in its entirety herein by reference,describes laser pulse-on-position technology. Alternatively, thecontroller 690 can control the system 800 based on timing data. Althougha control architecture (e.g., the motion stage 660, position sensor(s)680, controller 690, and defect map 695) is shown in FIG. 17 forcompleteness, control architecture are omitted from many of thefollowing drawings so as to not obscure the other components involved inthe illustrated laser processing systems.

Many different configurations, some offering more advantage than others,are possible. First, one can provide adjustable steering of one beam inXY space, while the other beam is fixed, as illustrated in FIG. 18, inwhich the fixed mirror 762 in one beam path has been replaced with adynamically adjustable XY beam steering mechanism 764, capable ofsteering the beam in both the X and Y directions, and a relay lens 770.In that case, one laser pulse, shown as the solid line, travels to thework surface 740 through a fixed optics path, while the second laserpulse, shown as a dotted line, contains an adjustable beam steeringmechanism 764, such as a fast steering mirror, in the optics path sothat the focused laser spot may be translated by a desired location inthe XY plane of the workpiece 740. Second, one can provide steering ofboth beams independently in XY space, as shown in FIG. 19, which depictsXY beam steering mechanisms 764 in both optical paths. This potentiallyoffers superior beam quality because smaller shifts of each beam canaccomplish a larger offset. For example, if a 40 μm separation betweenthe two beams is desired, then each beam can be shifted by only 20 μm indifferent directions. Smaller shifts result in less optical distortionand improved focused spot quality. Third, one can steer one beam in theX direction and the other beam in the Y direction, as shown in FIG. 20which depicts an X beam steering mechanism 766 in one optical path and aY beam steering mechanism 768 in the other optical path. Beam steeringmechanisms in the laser beam propagation paths (e.g., the beam steeringmechanisms 764) are preferably controlled by a control architecture(such as the one illustrated in FIG. 17).

In implementations employing a beam-steering mechanism 764, 766, or 768,such as in FIGS. 18-20, a relay lens 770 is useful. The relay lens 770works in conjunction with a beam-steering mechanism to adjust thetrajectory of the laser beam such that both beams strike the finalmirror 725 at the same spot. The different spot locations of the twobeams on the wafer 740 are attributable to the beams' different anglesof incidence into the focus lens 730.

Physical implementations of each of these configurations may occur manyways. For example, a two-spot system depicted in FIG. 21 is an alternateconfiguration providing XY steering of one beam relative to the otherbeam. That implementation, which uses two steering XY beam mechanisms764 rather than one with a relay optic 770, results in a system similarto the system depicted in FIG. 18.

Another implementation of the two-spot system 800 is illustrated in FIG.22, which shows a preferred embodiment in greater detail. In thatembodiment, the output beam of the laser 720 is preferably linearlypolarized. The beam goes through beam forming and collimating optics722, which may change the beam size and more importantly produce acollimated beam, before reaching a half-wave plate 724 and a beamsplitter 745. The beam splitter 745 is preferably a polarizer thatsplits the beam into two separate and orthogonally linearly polarizedcomponents A and B. Depending on the rotational orientation of the opticaxis of the half-wave plate 724, the ratio of power in beams A and B canbe adjusted continuously while the total power (A+B) is substantiallypreserved. For example, where it is desired that both focused spots onthe work surface be identical, the optic axis angle of the half-waveplate 724 can be adjusted such that the two spots on the work surfacehave the same power despite variations in the power throughput betweenthe two beam paths A and B.

Beam A out of the splitter 745 then goes through a switch 750, which ispreferably a fast switching device such as an AOM. Depending on thedesired switching speed or the construction of the AOM, beam formingoptics (not shown) may be employed immediately before and after the AOMto facilitate suitable beam size and divergence inside the AOM.Alternatively, other fast switches such as an electro-optic modulator(EOM) can be employed. Obviously, the switch 750 can be omitted ifindependent control of the two beams is not desired, in which case bothbeams will be switched on or off at the same time.

Continuing down the optical path A, beam size control optics 752 can beemployed to change the beam size such that the desired focused spot sizeis produced at the workpiece 740. For example, a programmable zoom beamexpander (ZBE) can be implemented to vary the output beam size andtherefore the final spot size at the work surface. Alternatively, otherbeam forming optics can be employed depending on the desired spotintensity profile and size.

In this embodiment, a suitable beam splitter 754 can be employed beforeor after the collimating optics 722 for power monitoring purposes. Forexample, in the incident path, a portion of the incident beam can bediverted to an incident detector 756 to monitor the magnitude andfluctuations in the energy of the laser pulses.

Optical signals reflecting off the features on the workpiece 740 cantravel backwards along optical paths of the system. Beam splitter 754can direct the reflected optical signals into reflected detector 758 formonitoring. Reflected and incident optical power levels are useful incalibrating the position of the focused spot, for example. During analignment process, the focused spots are scanned over alignment marks onthe work surface. Reflected power levels and position measurements areused to calibrate the position of the spot relative to the workpiece740.

In the implementation depicted in FIG. 22, the polarizing optics causethe signals reflected off the workpiece 740 to travel back up theopposite path. Thus the reflections produced by the incident beam A totravel back up the solid beam path B. Likewise, the reflection from theincident beam path B will travel back up the dashed beam path A. Thisresults in a crossing of the reflected signals. A preferred mode ofoperation when comparing incident and reflected signals is to usedetectors coupled to opposite beam paths. Such-comparisons may be usefulfor calibration and measurement purposes, such as determining the energyor properties of optical spots or performing alignment scans on theworkpiece.

The next major component in the beam path A is a beam scanning orsteering mechanism 760 that controls the focused spot location on thework surface. This is preferably a fast steering mirror that can movethe spot in both X and Y directions on the work surface, although twoscanning mirrors arranged in an orthogonal configuration can beemployed, each scanning in one direction only. The one-mirror approachis preferred because it can produce angular changes in both axes of thebeam while maintaining a stationary center of rotation at or near themirror center. Alternatively, other scanning devices, such as AOMs, canbe used for relatively small scan ranges but at high scan speeds.

In configurations with multiple scanning beams passing through a singlefocusing lens 730, it is desirable to place all beams at or near thecenter of the entrance pupil of the focusing lens 730 throughout thescanning range for optimum focused spot quality. This is especially truefor focusing lenses of high numerical aperture (NA) and with input beamsizes that nearly fill the entrance pupil, as commonly encountered inlink processing systems with spot sizes that are less than, for example,three times the beam wavelength. A common attribute of these high NAlenses is that the entrance pupil is located close to the lens, where itis very difficult, if not impossible, to fit physically all the beamscanning components. It is therefore advantageous to use relay lenses770 that can reproduce the scanning angle from each distant scanningmechanisms into the entrance pupil. The relay lens 770, which maycomprise multiple optical elements, is positioned between the steeringmechanism 760 and the beam combiner 765 downstream. Positioning of therelay lens 770 affects performance, as the center of the steering mirrorshould be located at the center of the entrance pupil of the relay lens770, while the exit pupil of the relay lens 770 coincides with theentrance pupil of the focusing lens 730. With this arrangement, the beamposition at the entrance pupil of the focusing lens 730 remainssubstantially stationary over scan, thereby maintaining optimum beamquality of the focused spot throughout the scan range.

Another desirable property of the relay lens 770 is that the state ofcollimation of the beam through the lens be preserved. In the preferredembodiment, the beam size and the magnitude of the beam angle relativeto the optical axis remain unchanged between the input and output of therelay lens 770. However, with a different design, the output beam fromthe relay lens 770 can have a different beam size and correspondinglydifferent angle while maintaining the desired collimation. For example,doubling the beam size out of the relay lens 770 is likely to half theoutput beam angle. This arrangement may be useful in certaincircumstance when it is desirable to reduce the scan angle range at thefocusing lens 730 to enhance the positioning sensitivity of the spot onthe work surface.

The relay lens 770 can be replaced by the addition of a scanning mirroror mirrors to the aforementioned optical train. With a minimum of twoindependent scan mirrors and by suitable manipulations of scan angles,it is possible to produce a scanning beam that is stationary at theentrance pupil of the focusing objective and with varying desired scanangles, thereby optimizing the spot quality throughout the scan range.The disadvantage of such an arrangement is that the total mirror scanangles are greater than compared with the relay lens case; this can be afactor in fast-scanning, high-resolution mechanisms where the scan rangeis limited.

For the second beam path B out of the beam splitter 745, the opticaltrain components can be substantially similar up to the beam combiner765 if independent switching and scanning is desired. Optionally, ahalf-wave plate 724 may be employed if necessary to produce the desiredpolarization properties. In the preferred embodiment, the output beamsfrom the two paths A and B with orthogonal polarizations are combined ina polarizer in such a way that the two output beams enter the entrancepupil of the focusing objective lens 730 at substantially the samelocation and direction when the scan mirrors are adjusted foroverlapping spots on the work surface. The beam combiner 765 can be, forexample, a cube polarizer or a thin-film plate polarizer, both of whichare commonly available optical elements. This arrangement has theadvantage of minimal power loss, although it is envisioned that otherbeam combiners such as diffractive optical elements or non-polarizationsensitive splitters may be employed here. Moreover, the input beams tothe combiner 765 may be non-linearly polarized.

After the beams are combined by the beam combiner 765, the mirror 725directs the beams toward the focus lens 730. The mirror 725 mayoptionally be a scanning mirror or an FSM, inserted into the opticaltrain for additional scanning. The center of scanning is preferablyplaced at the entrance pupil of the focusing lens 730, again for optimalfocusing. This scanning mirror may be used for beam positioning errorcorrection that is associated with the motion stages or otherpositioning error sources. This scanning mirror also may be used as analternative beam positioning device to one of the beam steeringmechanisms 760 for beams A or B. In that arrangement, the beam steeringmechanism 760 of beam B may be eliminated, for example, and the motionof the scanning mechanism 760 of beam A in conjunction with the motionof the mirror 725 produces the desired spot positions at the worksurface 740.

In most link processing applications, it is desirable to have focusedspots of nearly identical size and intensity profile. By measuring thesize of each spot and adjusting the beam size control optics 752 of thetwo beam paths, spots of nearly identical size can be created.Alternatively, the two beams can be recombined after the switches usinga polarizer and the combined beam sent through common beam size controloptics before they are split a second time. The use of common beam sizecontrol optics should ensure substantially identical focused spots atthe work surface.

Intensity profile(s) can be controlled by the adjustment of half-waveplate 724, through drive signals delivered to optical switches 750, orthrough the use of optional additional attenuation optics.

In a multi-beam system, it may also be advantageous to have all beamsfocus simultaneously and exactly on the same plane (parfocalityproperty) at or near the focal plane of the objective lens 730. Despitesubstantial similarity in the optical paths, normal tolerances in theoptical components may prevent perfect parfocality. It is thereforedesirable to introduce focus control optics 769 in each of the opticalbranches, although one of these can be omitted if the focus from thatbranch defines the focal plane. It is also possible to integrate thisfocus control function into the beam size control optics 752.

As the preceding examples illustrate, many different configurations andimplementations of multiple-spot systems are possible. Variations canarise with different physical implementations such as bulk optic orfiber optic implementations, the types of optical components used (whichare discussed in greater detail in another part of this document), theordering of optical components, the number of laser pulse sources, andthe desired configuration. Those skilled in the art will readilyappreciate that a multitude of optical configurations can result intwo-spot and multiple-spot laser processing systems.

A wide variety of optical components can be used to implement multiplespot laser processing systems. Described herein are many bulk opticcomponent choices from which these systems can be constructed. Otheroptions will be evident to those skilled in the art. The primarycomponents pertinent to this invention are laser sources, beamsplitters, beam switches, rotation generators, and beam altering optics.

The use of different lasers and different laser pulse properties inmulti-beam laser processing may favorably improve the processing ofsemiconductor link structures. Many different types of laser sources maybe employed or combined in multi-beam laser processing systems. Theselaser sources may include solid state lasers, such as diode-pumpedq-switched solid state lasers, including lasers containingrare-earth-doped lasants such as Nd:YVO₄, Nd:YLF, and Nd:YAG andvibronic lasants such as alexandrite, Cr:LiSAF, and Cr:LiCAF. Thefundamental wavelength output of these lasers may be converted toharmonic wavelengths through the well-known process of nonlinearharmonic conversion.

These laser sources may further include diode-pumped mode-locked solidstate lasers, such as, SESAM mode-locked Nd:YVO₄ lasers capable ofproducing pulsed picosecond laser output. Mode-locked solid state lasersmay include oscillator-regenerative amplifier and oscillator-poweramplifier configurations. The fundamental wavelength output of theselasers may be converted to harmonic wavelengths through the well-knownprocess of nonlinear harmonic conversion. The laser sources may alsoinclude chirped pulse amplification laser systems for the generation offemtosecond (fs) laser output or may alternatively include other pulsestretching and compression optics well-known to the art for the purposeof generating pulsed femtosecond laser output.

These laser sources may further include pulsed rare earth-doped solidcore fiber lasers and pulsed rare-earth-doped photonic crystal fiberlasers. Pulsed rare-earth-doped fiber lasers may include q-switched andoscillator-amplifier configurations. Further, a wide variety ofoscillators may be employed, including broad area semiconductor lasers,single-frequency semiconductor lasers, light emitting diodes, q-switchedsolid state lasers, and fiber lasers. The fundamental wavelength outputof these lasers may be converted to harmonic wavelengths through thewell-known process of nonlinear harmonic conversion.

Additional laser sources may further include semiconductor lasers, gaslasers, including CO₂ and argon-ion lasers, and excimer lasers.

A wide range of wavelengths, from about 150 nm to about 11,000 nm, canbe produced by the laser sources that can be included in multi-beamlaser processing systems. Depending on the laser sources employed,pulsewidths ranging from 10 fs to greater than 1 μs and PRFs rangingfrom pulse-on-demand to greater than 100 MHz can be produced at the timeof this writing. Depending on the laser sources employed, the pulseshape, energy per pulse or output power, pulsewidth, polarization,and/or wavelength may be tunable or selectable.

Laser sources with adequate energy per pulse output are desirable formulti-beam applications where the output from one laser source is splitand delivered to multiple workpiece locations. Many lasers currentlyemployed in link processing systems can produce adequate energy perpulse for multi-beam implementations due to the anticipated shrinkage indevice structure feature sizes.

Ultra fast lasers, which deliver numerous pulses in rapid succession toprocess a link, are also applicable to multi-beam laser processing. Inaddition to use in the system like any other laser source, thegenerating and blocking of pulses in a system employing an ultra fastlaser can be coordinated to allow different pulse sequences to bedelivered down each of the multiple beam paths. For example, more orfewer pulses may be permitted to pass down one of the beam paths fordelivery to a link. Pulses may also be delivered in bursts or deliveredalternating down the different beam paths. An offset or adjustment inthe laser spot location relative to the workpiece in one or more of themultiple beam paths can also be created by allowing a temporallydifferent set of laser pulses to reach the target links.

Beam splitters may be bulk optics such as polarizing beam splitter cubesor partially reflecting mirrors. AOMs, EOMs, and switchable LCDpolarizers may also be configured and driven to perform beam splitting.Alternatively, fiber optic couplers may serve as the beam splitter infiber-optic implementations.

Optical components for switching beams to allow pulses to propagate tothe work surface 740 or be blocked include: AOMs, EOMs, pockel cells,switchable LCD polarizers, mechanical shutters, and also high-speed beamdeflectors such as steering mirrors.

Beam steering mechanisms are typically in the class of rotationgenerators. Mechanical rotators include steering mirrors that may beactuated with piezoelectric, electromagnetic, electrostrictive, or otheractuators. Galvanometers, tilt wedges, and arrays of micromachinesmirrors also fall into the category of mechanical beam deflectors. Otheroptical elements that can steer optical beams include AOMs and EOMs.

For some applications, it may be possible to implement multi-beam lasersystems with fixed beam steering mechanisms instead of ones responsiveto input commands. Fixed or manually adjustable optics can be used toconfigure a link processing system to operate with focused spots thathave a relative position spacing that matches link spacings onparticular workpieces 740. Such systems would benefit from having somefixed paths that are used on X-axis link runs and other fixed paths usedfor Y-axis link runs.

A wide assortment of additional beam-altering optics may be included inthe optical paths. Similar and/or different elements may be used indifferent beam paths. These additional optic elements may includepolarizers, polarization modifiers, faraday isolators, spatial beamprofile modifiers, temporal beam profile modifiers, frequency shifters,frequency-multiplying optics, attenuators, pulse amplifiers,mode-selecting optics, beam expanders, lenses, and relay lenses.Additional optic elements may also include delay lines that can consistof extra optical path distance, folded optical paths, and fiber-opticdelay lines.

In the case of the full overlap configuration 570 or the partial overlapconfiguration 580 (FIG. 5), the implementation can be simplified. Forexample, FIGS. 23 and 24 are diagrams of systems 900A and 900B,respectively, producing the partial overlap configuration 580 of twolaser spots. The system 900A comprises a laser 720 producing a laserbeam having a series of pulses that pass through an X-axis AOM 761, aY-axis AOM 763, and optics 735 before reaching a workpiece 740. TheX-axis AOM 761 splits the laser beam incident at its input into twobeams directed in different directions along the X-axis; the Y-axis AOM763 does the same along the Y axis. Although typically only one of theAOMs 761 and 763 would be active at a given time (FIG. 23 shows theY-axis AOM 763 active), having both in series allows the system 900A tooperate on links having a lengthwise direction in either the Y or the Xdirection without having to reposition the workpiece 740. In place ofthe AOMs 761 and 763 any suitable beam-splitting device can be used. Inthe AOMs 761 and 763, the properties of the two output beams depend uponthe characteristics of a radio frequency (RF) control signal (notshown). More specifically, the displacement between the two output beamsis a function of the frequency of the RF signal, and the ratio ofenergies in the two output beams is a function of the power of the RFsignal. That energy ratio preferably varies between zero and one. As anoption, it is also possible to include a delay element 731, such as afiber-optic loop, to delay one beam relative to the other. Finally, theoptics 735 include such things as a final focusing lens and whateverother optical elements are desired.

The system 900B is an alternative implementation to produce twopartially overlapping beam spots. The system 900B includes a wave plate725 and a beam splitter 745, which splits a laser beam into two beams,which pass through respective diffractive elements 767 and 769. Thediffractive element 767 splits the laser beam at its input into twobeams directed in different directions along the X-axis, while thediffractive element 769 does similarly along the Y axis. Like the X-axisAOM 761 and the Y-axis AOM 763 in the system 900A, the birefrigentelements 767 and 769 provide flexibility to handle links extending ineither the X or the Y direction. The outputs of the diffractive elementspass through a combiner 765 and optics 735 to reach the workpiece 740.

Multiple laser sources may be used with multiple spot laser systems forgreater flexibility and performance in the processing of laser links.Different configurations offer different advantages. For example, FIG.25 shows one configuration utilizing multiple lasers 720-1 and 720-2, afirst mirror 722, beam combiner 723, and a second mirror 724. In onemode of operation, the multi-laser configuration can be used to increasethe effective laser repetition rate. By combining the multiple laserheads into a single output beam and sequentially triggering the laserheads to generate pulses, the effective laser repetition rate isincreased. Since the effective laser repetition rate is increasedwithout any increase in the repetition rates of the individual lasers,pulse properties are preserved. The pulse shape, pulse width, peak pulseheight, and pulse energy can all be maintained. Increasing the laserrepetition rate by driving a single laser at a high rate would increasepulse width and reduce available pulse energy. As an example of thistechnique, two 40 kHz lasers can be used to create a pulse trainoperating at 80 kHz with the same optical properties as a 40 kHz laser.

A multiple-laser configuration may also be operated to fire some or allof the lasers 720-1 and 720-2 simultaneously and have their outputpulses combined to increase the available pulse energy.

A multiple-laser configuration may also be operated to create opticalpulses with different optical properties that can be firedsimultaneously or with small time delays for temporal pulse shaping. Forexample, pulses from a laser with a fast rise time may be combined withpulses from a laser with a long pulse width to create combined pulsesthat have a fast rise time and long pulse width.

Lasers of different wavelengths can also be combined in a similarmanner. Multi-beam pulse shaping techniques previously described in thisdocument can also be applied to these lasers to further tailor the inputpulse train to a multi- or a single-beam link processing system. Forexample, a system equipped with lasers of IR and UV wavelengths couldselectively use a pulse from either laser source to process links.Alternatively, the different laser heads could deliver pulses withdifferent temporal shapes.

Combining continuous wave and pulsed lasers offers additional advantagein link processing systems. If the two beams overlap or have a knowndifference in focused spot locations, a continuous wave laser may beused for alignment and calibration and a pulsed laser may be used forlink processing. Such an arrangement can take advantage of the fact thata continuous wave laser is better suited for alignment and calibrationbecause it is always on and more stable than a typical pulsed laser.

Multiple laser heads may also be implemented where there are one or morelaser heads that are delivered to a focused spot. This configuration maybe repeated in a multiple-spot system such that each focused laser spothas a separate laser or lasers that provide pulses with which to processlinks.

The concepts described above can be generalized to many spots and manylaser heads: Semiconductor link processing systems can beneficially beconfigured using an arrangement of M laser heads to produce N beam pathsand K focused laser spots, where M, N, and K are integers greater thanor equal to one.

As noted above, multiple lasers may fire at the same time and/ordifferent times to combine or alternate between pulses. The lasers maybe of the same or different optical properties. And laser pulse trainsmay be further divided and delivered to multiple link structuressimultaneously or sequentially.

FIG. 26 depicts an optical system that combines pulse trains from Minput lasers into N output optical beams. That optical system can becreated as a functional unit of a link processing machine.

Many of these functional optical system groups can be combined into onelink processing system. The inputs to a beam combining optical systemmay be either laser heads or the output from another beam combiningoptical system. Similarly, the output from these optical subsystems maybe delivered to focusing optics for processing semiconductor links, ormay serve as the input to other beam combining optical subsystems.

The resulting mesh of laser heads that are interconnected throughoptical systems to multiple target links has great flexibility for (1)creating optical pulses with desired properties and (2) increasingsystem throughput by reducing the number of link runs required toprocess a product and increasing the link run velocity.

Implementation details presented above describe how to configure asystem for producing multiple spots where all the focused beams emanateonto the workpiece 740 through a single focus lens 730. However, it isalso possible to utilize multiple focus lenses. These multiple focuslenses can be arranged with an on-axis spacing, a cross-axis spacing, orboth.

Multiple final focus lens systems can be created with numerous lensconfigurations. The system can contain two or more lenses along theon-axis of a link run and/or along the cross-axis of a link run. Lensesmay be configured in a regular arrangement, a staggered arrangement, ora random arrangement. It is also possible to have an arrangement oflenses in a “+” plus configuration. A subset of the multiple focuslenses can be used for X-axis link runs and a different subset of lensescan be used for Y-axis link runs. The above mentioned lensconfigurations are a small subset of example configurations. Many otherlens arrangements are possible, each with different advantages.

Implementing multiple lenses with a cross-axis spacing allows processingmultiple link runs simultaneously. Thus, the number of times that thewafer must be passed underneath the focusing lenses is divided by thenumber of lenses. This can result in a dramatic throughput improvement.Additional advantages of laterally spaced spots were previouslydiscussed in Section III and are applicable to multiple-lens systems.

Implementing two or more lenses in an on-axis configuration alsoprovides throughput and hardware advantages. The on-axis spaced spotadvantages previously discussed in Section IV, such as multi-blow, areapplicable to multi-lens systems.

Spacing multiple lenses along the axis of link runs can also make eachlink run shorter. For example, two lenses spaced 150 mm apart will allowthe processing a 300 mm wafer with link runs that are at most 150 mmlong. Relative motion requirements to process a link run at the centerof the wafer are the wafer diameter divided by the number of lenses.

With this processing method, link run velocity is unchanged from thesingle-spot case, however a dramatic time reduction results due to theshorter link runs. An additional benefit is that the travel range of themotion stages may be reduced. A smaller stage will reduce stage cost andfootprint, and potentially increase the acceleration and bandwidthcapabilities of the stage.

Due to the size of each focus lens, and the short focal length used intypical processing systems, it is likely that the focused spots in amultiple lens system would have a large (on the order of inches)separation at the workpiece. However, it is possible to create amultiple focus lens system that produces overlapping spots. Systemsemploying smaller lenses (e.g., UV lenses of 2-3 inches in diameter) mayalso accommodate more lenses than systems employing larger lenses (e.g.,IR lenses of 3-5 inches in diameter) to achieve small spot sizes. Forexample, a UV system capable of processing 300 mm wafers may be able toemploy up to about 6 lenses.

FIG. 27 shows one implementation of a multiple lens semiconductor linkprocessing system. Many alternative configurations are possible. Such asystem can be positioned above the workpiece 740 (shown as a dashedline) that is on the XY motion stage 260 (not shown in FIG. 27) toperform link runs.

The multiple-lens system in FIG. 27 includes a single source laser 720that produces a pulse train with beam splitters 745A-745D and a mirror775 for delivering optical pulses to each focus lens 730A-730E. Optionalsteering mirrors 764A-764E or fixed mirrors are included before eachfocus lens. Independent AOMs 752A-752E or other switches are used toblock pulses that are not meant to process links. Each lens 730A-730Ealso preferably has a focus mechanism and a fast steering mirroradjustment of the focused spot beam waist location.

In a multiple-lens system, it is advantageous to use an XY beam steeringdevice such as an FSM or steering mirror before each focus lens to makesmall displacements of the focused spots to precisely position eachfocused spot at the desired target link. Use of the steering mirrors764A-764E can correct for minor irregularity in the alignment orplacement of each beam path and/or focus lens. These steering mirrorscan also be used to compensate for (1) wafer rotation relative to thearrangement of lenses, (2) layout offsets, rotations, geometricirregularities, calibration factors, scale factors, and/or offsets thatmay differ across the wafer, and (3) dynamic or other errors that mayhave a different impact at each focused spot. In short, steering mirrorsmay help of any multiple-lens processing system to properly position allspots at the desired location on the workpiece at the correct instantwhich may require unique calibration parameters and/or compensation ofeach focused spot location.

The advantage of using multiple lenses may at times be limited bypacking loss. Packing loss occurs because not every lens is always abovea region on the workpiece 740 that requires processing. For example,referring to FIG. 27, when processing near the edge of the workpiece740, the lenses 730A and 730E at the top and bottom of the figure mayhave their focused spots land off the workpiece 740. Because not allfocused spots are usable in such a case, some inefficiency can result.

The asymmetrical configuration of lenses in the multiple-lens system ofFIG. 27 (with alignment along the Y-axis) makes it natural to use anasymmetrical motion stage and different processing modality for X and Ylink runs. The wafer 740 is moved about with a planar XY motion stagethat has a long travel axis and a short travel axis. For example, 300 mmtravel in the X direction is required, but only about 60 mm travel in Yis required. This contrasts with the motion stage in present single-lenssystems where a 300 mm travel in both axes is provided. Reducing thetravel requirements in the Y axis would reduce the motional mass andfootprint of the stage.

Use of a motion stage with different X and Y performance characteristicsthat are matched with the X and Y processing modalities is desirable andoffers additional advantages. One such motion stage is a stacked XYstage that has inherent properties well suited for use in the system ofFIG. 27. In an implementation of stacked motion stages, the X-axismotion stage carries the Y-axis motion stage. In such a configuration,the X-axis motion stage typically has less acceleration and bandwidthbecause it carries the mass of the Y-axis motion stage, yet it may havean extended travel range. The lighter Y-axis motion stage can delivergreater acceleration and bandwidth. The Y stage mass can be furtherreduced if only a short travel range is required.

This combination of properties is well suited for use with themultiple-lens processing system of FIG. 27. A preferred configuration isto align the X-axis motion stage with the optics table such thatcross-axis parallelism reduces the number of link runs. The Y-axismotion stage is aligned with the optics table for on-axis parallelism.The many shorter link runs in the Y-axis are processed with the higherperformance Y stage, and the lower performing X-axis is used to processfewer link runs.

Because typical DRAM wafers are often asymmetrical in the number of linkruns and link densities along each axis, there may be a preferredorientation of the workpiece 740 to a multi-beam processing system.Typical DRAM wafers have one processing axis with many more link runsand greater link density. The other axis has fewer link runs, butsparser links with more gap profiling opportunities. In that case, adesirable processing configuration orients the wafer such that the axiswith many dense link runs is processed with the slower axis (the X-axisin the stacked stage described above) using cross-axis parallelism. Thisreduces the required number of link runs. Because there is lessopportunity for gap profiling in that direction, due to link density,the lower performance motion stage in that direction is appropriate. Thefaster axis is then used to process the sparse link runs and it can takeadvantage of on-axis parallelism to quickly process many link runs withmore opportunities to benefit from gap profiling.

An alternative way to use the multi-lens system in FIG. 27 is to processall link runs as either X- or Y-axis link runs. This takes advantages ofthe on-axis or cross-axis advantages of multiple lenses in eitherconfiguration. In order to process all the link runs as either X or Yaxis link runs, it would be necessary to rotate the wafer. This can beaccomplished by designing a rotation mechanism into the chuck or byremoving the wafer from the chuck, rotating it with a rotatingmechanism, and then reloading the wafer onto the chuck surface. Toreduce the time required to rotate the wafer, one can include amechanism in the system that can remove a wafer from the chuck andquickly place a different wafer on the chuck. While one wafer is beingprocessed, the other wafer can be rotated.

One advantage of processing all link runs in the same direction is thatthe motion stage can be optimized to process in that orientation. Forexample, if all link runs are done as short Y-axis runs, the Y axis canbe optimized for high acceleration and bandwidth and low mass. In thatcase, however the X-axis requirements can be relaxed in comparison topresent systems since it need only laterally advance between link runsand make small motions for X alignment scans. High accuracy may still berequired in the X axis, however high speed and acceleration may not beas important.

Semiconductor ICs are typically fabricated as a regular grid ofnominally identical rectangular die disposed upon the wafer. All ofthese die contain the same arrangements of links and link banks, andtherefore may be processed with a similar pattern of link runs. However,the specific fuses to be severed on each die are the result of a testingprocess and therefore usually differ. The regular arrangement ofidentical die on wafers motivates a preferred arrangement of lenses in amultiple-lens processing system. It is natural and desirable to adjustthe focus lens and moreover the focused spot spacing to be integermultiples of the die dimensions. Of course, small correction factors mayneed to be applied using a beam steering mechanism to account forcalibration, scaling, and orientation differences such as minorrotations of the wafer. Spacing the lenses and/or focused spots in thismanner allows each spot to simultaneously impinge upon the samecorresponding links and link banks of different die. Processing two moredie at the same time and processing the same corresponding links on twoor more die at the same time using a multi-beam system are preferredmodes of operation.

For example, suppose that each die has four link runs of types A, B, C,and D that need to be processed in the X direction. By adjusting therelative spacings between focused spots so that all lenses areprocessing the same links in link runs of type A at one time, thensimply adjusting the wafer location in the cross-axis direction usingthe XY stage allows processing of all of the type B link runssimultaneously. One advantage of this technique is that coarse lens andfocused spot locations can be adjusted once for each link run direction.Adjustment between link runs to ensure that all focused spots would beable to fall upon links is unnecessary. A second advantage of thistechnique is that it is much easier to produce a joint velocity profile,and there will be more opportunities for gap profiling. This arisesbecause profitable gaps may occur in the same location of a type of linkrun on each die. Triggering the laser with purely cross-axis spacings isalso easier because focused laser spots hit the same corresponding linkon multiple die at the same time.

It may also be desirable for the lens spacing to match an integermultiple of the wafer mask size (focused mask size). This allows for diethat are patterned on the wafer in one patterning step to all beprocessed by the same lens. Small step and repeat errors that occurredduring the mask steps can then be calibrated out more easily.

A multiple-lens system that simultaneously processes multiple differentdie may have fewer opportunities for gap profiling in comparison withthe single-lens systems. Single-lens systems have an opportunity to savetime by gap profiling whenever it is unnecessary to process links on adie because they are unrepairable or perfect. Since multiple differentdie may be simultaneously processed with a multiple-lens system, therewill be opportunities to skip over a complete die. However, the timesaved by using multiple spots will be greater than the time lost due toless gap profiling. One advantage of a fast system that uses less gapprofiling is that less heat is generated by the motion stage. The motionstage specifications may even be relaxed resulting in a lower cost, amore easily produced system, and a more compact system.

In a multiple-lens processing system, it is desirable to include amechanism by which the spacing of lenses can be adjusted by severalmillimeters. This allows the lens spacing to be adjusted to matchinteger multiples of the dimensions of die on different customerproducts. Perfect placement of the focus lenses is not required as abeam steering mirror, such as a FSM (fast steering mirror) can fine tunethe spot locations once the lenses have been mechanically adjusted.

A final aspect of multiple focus lens systems is that it is possible toproduce multiple spots that emanate from each of the focus lenses. Bydoing so, link run velocity can be further increased and/or the numberof required link runs can be further reduced, hence system throughput isfurther improved. The aforementioned advantages of multiple spots from asingle focus lens can be applied to multiple spots from multiple focuslenses. These combined advantages are possible with on-axis and/orcross-axis spaced focused lenses, each delivering on-axis and/orcross-axis focused spots to the workpiece.

Other important aspects of multiple-spot processing are the softwaremethodologies for determining links to be processed in parallel, thejoint velocity profile, and conveying link run data to the computers orcircuitry controlling the hardware.

For multiple-spot link runs where a nominally fixed offsets existsbetween the spots, it is not necessary to transmit all of the linkcoordinates that are to be processed for each spot. It is suffice todesignate a “master spot” and convey, from the system control computerthat designates link runs to the hardware controlling computer, theoffsets of other spots relative to the master spot. Then the ‘masterlink’ coordinates to be processed by the master spot can be conveyedalong with one data bit for each spot designating if the switch 750 foreach beam should transmit or block pulses corresponding to each masterlink. This would dramatically reduce the amount of data that needs to betransmitted. Only one bit of information would need to be transmittedfor each additional spot to be processed instead of link coordinates,each of which is a multiple byte number.

VIII. Error Correction

U.S. Pat. No. 6,816,294 describes the use of a FSM to shift the positionof a focused laser spot to correct for relative positioning errors thatoccur in an XY motion stage. The art described in that patent is fullycompatible with the multiple laser beam processing systems describedherein. As described therein, a laser beam is directed toward a targetlocation on a workpiece in response to a coordinate position command. Inresponse to that command, the XY motion stage positions the laser beamover the coordinate position on the workpiece. The system also sensesthe actual position of the workpiece relative to the coordinate positionand produces an error signal indicative of the position difference, ifany. A servo control system associated with the XY motion stage producesa position correction signal to compensate for the difference, therebymore accurately directing the laser beam to the target location. Asimilar servo control system can be utilized to direct multiple laserbeams to multiple target locations. For example, in a two-spot system,the relative positioning errors due to XY motion stage error will impactboth spots equally, and the inclusion of a final XY beam steeringmechanism 772 may be used to redirect and correct those errors for themultiple-spot case, as shown in FIG. 28. Errors caused by XY stagerotation will not impact both spots equally, however any sensed rotationerrors can be corrected in a similar manner using a rotation coordinatetransformation and the two beam steering mechanisms 764.

Moreover, a system configured with steering mechanisms on one or morebeam paths and also a final steering mirror offers additionalflexibility for commanding spot motion and correcting errors. All of theXY beam steering mechanisms and any final FSM beam steering mechanismmay be collectively used to impart desirable motion of the spots. Forexample, the final XY beam steering mechanism could move both spots by+20 μm in the X direction, and then the independent beam steeringmechanism could move one spot by +20 μm in the X direction and one spotby −20 μm in the X direction. The resulting configuration has one spotthat is unchanged from the initial position, the other spot displaced by+40 μm, and no actuator imparting more than 20 μm of motion. Oneadvantage of such a configuration is that the amount of displacementimparted by any beam steering mechanism can be reduced. Furthermore, inconcert with generating spot offsets, all the beam steering mechanismscan also work together to compensate for errors.

An additional advantage of the above configuration arises when theactuators have different performance specifications. For example, someactuators may have large travel range but limited bandwidth. Otheractuators may have very high bandwidth but limited range of travel.Selectively allocating the frequency content and range of the desiredbeam steering commands to match the different actuators can result in asystem that has a large travel range and also fast response necessaryfor error correction and command offset. Placing the beam steeringmechanisms that will impart larger position offsets near the entrancepupil of the focusing lens, and those that impart smaller offsetsfurther from the focusing lens can also result in less distortion of thefocused spots.

With some optical configurations, an additional FSM XY beam steeringmechanism may be unnecessary for correcting XY stage error. For example,if both beams have steering mechanisms allowing them to shift in both Xand Y directions (e.g., as shown in FIG. 19), and if these steeringmechanisms have sufficient bandwidth and range to correct stage error,then a final output FSM for error correction is redundant. The steeringmechanism commands used to shift the two spots relative to one anothercan be combined with the commands necessary for error correction. Theresultant steering mirror motions correctly position the spots relativeto one another at the wafer surface and also correct for relativepositioning errors.

Having independent steering mirrors for the two beams additionallyallows for the correction of errors or calibration and scale factorsthat impact the ability to position each spot and the desired targetlocations. For example, wafer fabrication errors may cause slightlydifferent scale factors or rotations of different die on the wafer, andbeam steering can correct for these differences. Alternatively,optic-table resonances, optic vibrations, thermal drift of opticcomponents, or other changes in the system could cause differentrelative positioning errors between each focused spot and target linkstructures on the workpiece 740. Independently driven steering mirrorscoupled to sensors that detect position errors can make independentcorrections to the different beam paths. Such position sensors could beoptical encoders, interferometers, strain gauge sensors, inductiveposition sensors, capacitive position sensors, linear variabledisplacement transformers (LVDTs), position sensitive detectors (PSDs),sensors or quad photodetectors to monitor beam motion, or other sensors.Thus, using two steering mirrors provides more flexibility and benefitthan a single steering mirror for error correction and calibration.

Other sources of positioning errors not noted in the above-referencedpatent application can also be corrected using steering mirrors. Forexample, the pointing stability of the laser, AOM switches, andcomponents in the laser rail can be detected using optical or mechanicalsensors, and corrective action taken by the system's steering mirrors.

FIG. 29 shows an example of using the independent XY beam steeringmechanisms 764 to: (1) create offsets of one focused spot relative tothe other focused spot using relative offset commands; (2) correct forpositioning errors of the focused spot relative to the workpiecedetected in an XY stage servo system 784; and (3) correct for additionalrelative positioning errors detected with other position sensors. Inthis figure, PSDs 780 are used to measure beam motions resulting fromlaser pointing stability, AOM pointing stability, and mounting ofoptics. The errors measured in each beam path due to AOM pointingstability and motion of optical elements may be different in each beampath, so the figure depicts independent measurement, signal processing,and commands being delivered to the XY beam steering mechanisms 764.

It should be noted that beam steering mechanisms may preferably beadjusted before the start of link runs and also during the performanceof link runs to create and maintain the desired relationship betweensbetween focused spot locations and target link locations. For example,these adjustments may compensate for system errors and also differentcoordinate systems, calibration parameters, scale factors, and offsetsthat may apply to links in different locations about the wafer.

The separation distance between the spots in a multiple spot processingsystem may also require more accurate wafer positioning. This needarises due to abbé offset errors, which are translational positioningerrors attributable to small angular offsets over a significant leverarm. Single spot machines only require that the focused beam impingeupon the correct point on the workpiece when a laser pulse is triggered.This can be achieved with pure XY translation of the workpiece relativeto the focused spot, even if the workpiece has small rotation errors.Multiple spot machines require that all spots impinge upon the properlocations of the workpiece simultaneously. For a fixed configuration ofspots, small rotation errors will prevent all spots from simultaneouslyimpinging upon the correct workpiece locations. This is particularlytrue of multiple lens implementations where the spot separation islarger, however it applies to single-lens implementations as well.Control, and in particular feedback control, of the workpiece relativeto the multiple focused spots in more than two degrees of freedom ismore important in multiple-spot processing systems than in single-spotprocessing systems. This control is relevant to planar motion involvingX, Y, and theta (yaw) coordinates of the chuck and wafer, as well asfull three-dimensional control further involving Z, pitch, and roll.

Sensing mechanisms may be useful to control and correct for errorsincluding abbé offset errors in multiple spot systems. For example,putting an interferometer or other sensors in line with each lens in amultiple lens system can be a useful technique for error reductionbecause the positioning error near each lens can be detected.Furthermore, a central processor or FPGA can be employed to combine datafrom many sensors and to determine the geometric relationships betweendifferent system components such as: focused spot locations, steeringmirror positions, beam path position, lenses, chuck, links etc. Havingdetermined error between desired and measured the geometricrelationships between components, positioning errors can then bemitigated. This may be through multi-dimensional control of theworkpiece, chuck, stage, or other system components. Errors can also becompensated for using one or more FSMs to move the spots in the on-axisand cross-axis direction.

The geometric relationship between components is also useful for lasertriggering. If one laser is used to generate multiple spots, eitherthrough one lens or multiple lenses, the system can trigger laseremissions based upon position estimates or measurements such that one ormore spots will impinge upon the workpiece at the desired location(s).Triggering the laser at the estimated position(s) or time(s) thatminimizes the average error between each of the multiple focused spotlocations and the desired blow locations is one preferred pulsetriggering method. Similarly, if multiple lasers are used to generatemultiple spots, each laser can be triggered at the time(s) orposition(s) that minimizes the error between the focused spot locationand the target workpiece location. Other pulse triggering methods mayalso be implemented.

IX. Calibration, Alignment, and Focusing

Use of multi-beam path link processing systems can require calibrationof the beam energy parameters and spot position parameters for eachlaser beam. One way to accomplish energy calibration, as shown in FIG.30, is to tap off some optical power from each beam and use independentpulse detectors 790 to sense pulse properties, such as pulse energy,pulse height, pulse width and possibly others. Having sensed the opticalparameter(s), configurable hardware in the beam path or laser can beused to make adjustments. In one implementation, information from thepulse detectors 790 can allow the independent tuning of configurableattenuators 792 in each beam path for energy control. The attenuators792 may be standard optics, AOMs or other attenuators. Feedback from thepulse detectors 790 can also be used to modify the generation of thepulse in the laser. This offers additional advantage when multiple lasersources are used.

Performing system position calibration with multiple laser spots issimilar to present single-spot calibration. However, the Z-heightrelationship between each focused beam waist and each target linksshould be ascertained, as well as the XY position relationship betweenthe focused spots and the target links. Both of these relationships canbe determined by scanning alignment targets on the wafer. This scanningprocess involves delivering either continuous wave or pulsed opticalenergy to the surface of the wafer and laterally scanning the XY stagesuch that the light reflects off alignment targets with knowncoordinates on the wafer. Monitoring the amount of energy reflected fromthe targets and the stage position sensors allows the position of thelaser spots relative to the alignment targets to be determined withprecision. These monitored signals also allow determination of the spotsized with the present Z height separation between the lens and thealignment structure. A system for doing so is illustrated in FIG. 31, inwhich a beam splitter 794 and a reflected light detector 798 arearranged to detect the reflected signal. A quarter-wave plate 796 mayoptionally be employed to produce circularly polarized light output todeliver to the work surface 740.

To focus in a multiple-spot system, a target is scanned at several focusheights and measurements of contrast or the spot size at these focusheights are used to predict and iteratively refine the focused beamwaist. Because a multiple-spot system involving a single lens has onlyone lens-to-link structure or alignment target separation at a time, itmay be necessary to pre-align all of the focused spots of a multi-spotsystem so that they all have substantially the same focus height. Onemethod for doing so involves directing multiple laser beams onto targetsat one or more focus depths, taking focus depth measurements for thevarious beams, determining relative focus depth differences based onthose focus depth measurements, and adjusting the laser beam's paths inresponse, preferably to reduce the relative focus depth differences.That process can be repeated iteratively or by means of a feedbackcontrol system to achieve relative focusing pre-alignment. Thereafter,focus in a live wafer processing environment can be accomplished usingjust one of the focused laser spots. Focusing may be accomplished with asingle target in a focus field, or with multiple targets, such as threeor four targets, in a focus field. Focus height distances at the XYlocations positions within the focus fields are then computed from thefocus heights at the different focus target locations.

Focus in a multiple-spot system may also be enhanced by adding or movinga focus control optic 769 (FIG. 22) to offset one or more focused spotbeam waists from other focused beam waists in the Z direction.

In addition to being a useful independent focus mechanism, the focuscontrol optic 769 can impart a known Z focus offset of a focused beamwaist relative to other spots to enhance the focus methodology. Byscanning an alignment target with these two or more Z-offset spots, theZ direction that must be traveled to achieve focus is known. Three ormore Z-offset spots can be used to predict not just the focus direction,but also the distance to focus.

Another focusing technique involves small range of travel focusadjustors on each lens and a single coarse Z adjustment that can bealigned to approximately one wafer thickness and locked in place. Thisis preferably implemented on a system with a substantially flat andlevel chuck so that the lenses do not have to be shifted up and down tocorrect for wafer tilt while processing link runs. This greatly reducesthe amount of focusing work that must be done. Focus then only has totrack small (generally less than about 10 μm) deviations that occur dueto dust particles under the wafer or the chuck not being flat. Becauseeach lens may focus upon a different part of the chuck, a piezoelectricactuator can be implemented on each lens to allow it to be movedvertically by a small amount to adjust focus. Focus can be adjusted bythese piezo actuators so that the focused beam waist tracks the localwafer topology under each lens. Of course, alternative implementationsof this focusing technique are possible such as using voice-coil orother actuators rather than piezoelectric actuators.

One alignment procedure for a multiple-spot system involves determiningthe position of all the spots relative to alignment targets and also anyZ height dependency of this relationship. In the simplestimplementation, an XY alignment target is first scanned and measured byall of the spots in the system to determine the XY and potentially Zoffsets of these spots relative to one another. Then, the relativeoffsets may also be measured at different focus heights. This proceduremay be performed on a single target, or many focus targets at differentlocations on the wafer, or on a calibration grid. The informationgathered about the relative positioning of the spots at workpieceprocessing locations can be processed by one or more computerscontrolling the machine to calibrate and correct for differences in spotlocations when processing different areas of the wafer.

Having characterized the multiple spots relative to one another, waferXY alignment in different alignment fields can be implemented in amanner analogous to the single-spot system alignment. A target ortargets can be scanned to determine the geometric relationship between afocused spot and the target link structures, and a known mapping betweenthe spot locations can be applied to precisely determine the position ofthe rest of the system's focused spots. Then the XY beam steeringmechanisms and focus offset mechanisms can be sent positioning commandsto precisely position all of the focused laser spots at the desiredlocations for link runs and link run segments. This is preferablycarried out by creating three-dimensional reference surfaces whichdefine laser-to-workpiece calibrations in a region of the workpiece.Target link coordinates and the trajectory commands of stages, beamsteering mechanisms, and focus offset mechanisms can be generated fromCAD data of link blow locations, the reference surfaces, and anyadditional calibration information.

Some XY and focus calibration can be performed with only one of multiplespots on at a time. However there are other procedures where it isadvantageous to scan targets with multiple spots that are simultaneouslydelivered. For example, scanning an XY alignment target using all of thespots at the same time can verify that all spots are focused and thatthe relative offsets between spots have been removed with the XY beamsteering mechanisms through the calibration procedure. Reflectionsignals off of the scanned target would then appear to have thereflection signature of a single spot of tight focus. If any of thebeams are not properly aligned or are out of focus, then multiplepossibly overlapping reflection signatures will be observed, or thereflection signature of large spots superimposed with small spots may beobserved.

Another calibration procedure using multiple spots simultaneouslydelivered to the wafer uses an averaging technique to improve thequality of scan measurements. This technique is illustrated in FIG. 32.If the offset relationship between two spots is known and can beprecisely set, then two (or more) spots can be set up to have a smalllateral offset (e.g., a couple microns) along the axis that an alignmenttarget 810 will be scanned. Then a single scan of the alignment target,collecting reflected sensor data and stage position data, can be used todetermine the locations of the two spots. This information can becombined with the commanded spot offsets to determine the targetlocation with enhanced accuracy by averaging the two spot locations.This technique can be used to refine the accuracy of the spots relativeto one another in the scan direction. As an example, assume that theoffset distance in the scan direction is 5 μm. Assume further thatscanning of spot 1 over the alignment target 810 produces a maximumreflection intensity when the X position is 10,005.020 μm, and thatscanning of spot 2 over the alignment target 810 produces a maximumreflection intensity when the X position is 10,000.000 μm. Then, aftertaking into account the known offset and then averaging the two positionmeasurements, the resulting position would be 10,000.010 μm. Becausethat average is based on more data than a single measurement, it is amore reliable result.

In a system that can determine which reflections were caused by whichincident focused spot, it is possible to practice this averagingprocedure with fully overlapping spots. Time slicing and exploitingdifferent spot properties such as polarization or wavelength are sometechniques by which a reflected spot can be associated with an incidentspot. These techniques may be useful when the spots are partiallyoverlapping or fully overlapping such that the relative offset is zero.

In the second case depicted in FIG. 32, the two scanned spots have bothon-axis and cross-axis offset. That provides two estimates of thelocation of the alignment target 810 with measurements made at differentpoints along the alignment target 820. These multiple measurements areuseful for determining absolute positioning on the wafer even when thealignment target 820 is not uniform.

Next, since the beams of a multiple spot system may be equipped with XYbeam steering mechanisms, these mechanisms, rather than the XY stage,may be used to scan the focused spots across alignment targets 810. Thenthe calibration routine correlates reflected signal energy off ofalignment targets 810-820 with the sensed XY beam steering mechanismposition and combines this with the XY stage position to determine spotpositioning. Since independent XY beam steering mechanisms can be put ineach of the beam paths, it is possible to independently scan XYalignment targets 810-820 with different focused spots. One target canbe scanned in X while another alignment target 810-820 is scanned in Ywith an appropriate method for determining which is the X signal andwhich is the Y signal. This can be done by dithering the power in thespots at specific frequencies using an AOM or other attenuator to changethe energy, and then using the frequency information to determine whichreflected signal comes from each spot. Alternatively, scanning thealignment targets 810-820 with spots moving at different velocities canbe used to associate components of a reflection signal with a specificspot. Spots can be also be time-sliced or modulated at a high rate suchthat only one spot is on at time. Then reflection signals can bedirectly separated using time slices to allow scanning multiple targets,or an X and a Y target simultaneously. Separation based upon an opticalproperty such as polarization or wavelength may also be appropriate forsome implementations.

If multiple laser sources are used on a semiconductor link processingsystem, proper alignment will result in the highest quality linkprocessing. One technique for alignment of multiple laser heads entailsproducing continuous wave or pulsed emissions from laser heads,measuring the propagation of beams relative to one another, andadjusting the beams to a desired overlap or relative position. Measuringthe beams relative to one another may be done by scanning alignmenttargets 810-820 on wafers using the focused laser spots or it mayinvolve placing PSDs or other optical detectors in the beam paths atdifferent locations. An alternate technique is to place a PSD alignmenttool into the beam path in place of the final focusing lens. Then beampositions can be measured while using the Z stage to change the positionof the PSD, and optical elements, such as tilt plates and mirrors can beadjusted to correct beam positions. Measurement of beam or focused spotlocation may occur with all of the laser heads emitting individually orsimultaneously.

One desirable beam alignment is such that the emissions from each laserhead precisely overlap. Thus, the resulting single-beam system wouldhave focused beam waists in the same position regardless of which laserhead produced the pulse. Likewise, a two-beam system would produce twofocused spots.

Another desirable beam alignment is to introduce an intentional on-axisand/or cross-axis relative offset of the focused spots produced bydifferent laser heads. Such an offset may be implemented so that pulsesfrom one laser head impinge upon one row of links while pulses fromother laser heads impinge upon other rows of links.

The alignment of the laser beam paths may be adjusted during machineset-up and then not require further adjustment. However, there may besituations, such as to correct for thermal drift of focused spots, wheredynamic or periodic beam adjustment is desirable. Actuators may beplaced in the system for beam adjustment actuators and a control systemcan be put in place for configuring these actuators based upon scan datafrom alignment targets 810-820 or PSD measurements of beam position.

Actuators may also be used to reconfigure the alignment of beamsproduced by the different laser heads at times during wafer processing.For example it may be desirable to shift the positions of focused spotsemanating from different laser heads between the processing of X and Yaxis link runs, or between the processing of link run segments thatrequire a different spacing. Furthermore, when processing with multiplespots through the same lens, it may be desirable to make smalladjustments in the relative or absolute positions of the spotsthroughout a link run. For instance, there may be some dependency offocused spot XY position based upon Z height. If the beams are sloped,focusing at a different height due to a sloped chuck or changes in chuckand wafer topology may cause the spots to wander. Such errors can becorrected by using multiple beam actuators and/or beam steeringmechanisms.

The methods and systems illustrated and described herein (e.g., thecomputation of a joint velocity profile) can exist in a variety of formsboth active and inactive. For example, they can exist as one or moresoftware programs comprised of program instructions in source code,object code, executable code or other formats. Any of the above formatscan be embodied on a computer-readable medium, which include storagedevices and signals, in compressed or uncompressed form. Exemplarycomputer-readable storage devices include conventional computer systemRAM (random access memory), ROM (read only memory), EPROM (erasable,programmable ROM), EEPROM (electrically erasable, programmable ROM),flash memory and magnetic or optical disks or tapes. Exemplarycomputer-readable signals, whether modulated using a carrier or not, aresignals that a computer system hosting or running a computer program canbe configured to access, including signals downloaded through theInternet or other networks. Concrete examples of the foregoing includedistribution of software on a CD ROM or via Internet download. In asense, the Internet itself, as an abstract entity, is acomputer-readable medium. The same is true of computer networks ingeneral.

The terms and descriptions used above are set forth by way ofillustration only and are not meant as limitations. Those skilled in theart will recognize that many variations can be made to the details ofthe above-described embodiments without departing from the underlyingprinciples of the invention. For example, structures other thanelectrically conductive links on a semiconductor substrate can beprocessed with multiple laser spots. As another example, not all linkprocessing is for the purpose of severing a link so it does not conduct;sometimes the purpose of the laser radiation is to make an otherwisenon-conductive “link” conductive or to otherwise change a link'sproperties. The scope of the invention should therefore be determinedonly by the following claims—and their equivalents—in which all termsare to be understood in their broadest reasonable sense unless otherwiseindicated.

1. A method for selectively irradiating structures on or within asemiconductor substrate using a plurality of laser beams, the structuresbeing arranged in a row extending in a generally lengthwise direction,the method comprising: generating a first laser beam that propagatesalong a first laser beam axis that intersects the semiconductorsubstrate; generating a second laser beam that propagates along a secondlaser beam axis that intersects the semiconductor substrate; directingthe first and second laser beams onto distinct first and second spots onrespective distinct first and second structures in the row, the secondspot being offset from the first spot by some amount in a directionperpendicular to the lengthwise direction of the row; and moving thefirst and second laser beam axes relative to the semiconductor substratealong the row substantially in unison in a direction substantiallyparallel to the lengthwise direction of the row.
 2. The method of claim1, further comprising: selectively blocking the first laser beam fromreaching the semiconductor substrate; and selectively blocking thesecond laser beam from reaching the semiconductor substrate.
 3. Themethod of claim 1, wherein the first laser beam irradiates a givenstructure in the row for a first time, and the second laser beamirradiates the given structure later for a second time.
 4. The method ofclaim 1, wherein the a selected structure in the row is irradiated byeither the first laser beam or the second laser beam but not both,whereby the moving step can occur at a greater speed than would occur ifonly a single laser beam were utilized to irradiate the structures inthe row.
 5. The method of claim 1, wherein the first and secondstructures are not adjacent.
 6. The method of claim 5, wherein there ismore than one structure between the first and second structures.
 7. Themethod of claim 6, wherein the number of structures between the firstand second structures is an even number.
 8. The method of claim 1,wherein the first and second spots are separated by a distancesufficient to avoid a deleterious concentration of energy absorbed bythe semiconductor substrate between the first and second spots.
 9. Themethod of claim 1, further comprising: generating a third laser beamthat propagates along a third laser beam axis that intersects thesemiconductor substrate; and directing the third laser beam onto astructure in the row.
 10. The method of claim 1, wherein the first andsecond laser beams have respective first and second sets of opticalproperties, and wherein the first and second sets are different from oneanother.
 11. The method of claim 1, wherein the steps of generating thelaser beams comprise: generating the first and second laser beams fromtwo respective lasers.
 12. The method of claim 1, wherein the steps ofgenerating the laser beams comprise: generating a single laser beam froma single laser; and splitting the single laser beam to form the firstand second laser beams.
 13. The method of claim 1, wherein the steps ofgenerating the first and second laser beams are commenced based upon atrigger signal.
 14. The method of claim 13, wherein the trigger signalis generated based upon a timing signal.
 15. The method of claim 13,wherein the trigger signal is generated based upon a comparison of oneor more desired target locations and the positions of one or more thefirst and second laser beam axes on the semiconductor substrate.
 16. Themethod of claim 1, further comprising: during the moving step,dynamically adjusting the relative spacing between the first and secondaxes.
 17. The method of claim 16, wherein the adjustment of the relativespacing is in the lengthwise direction of the row.
 18. The method ofclaim 1, wherein the moving step comprises: moving the laser beam axes.19. The method of claim 1, wherein the moving step comprises: moving thesemiconductor substrate.
 20. The method of claim 1, wherein thestructures comprise electrically conductive links and the irradiation ofa link results in severing that link.
 21. The method of claim 1, whereinthe structures comprise potential electrically conductive links and theirradiation of a link results in making an electrical connection in thatlink.
 22. The method of claim 1, wherein the first and second laserbeams are pulsed laser beams.
 23. The method of claim 22, wherein thefirst and second laser beams are pulsed simultaneously on the first andsecond structures respectively.
 24. A semiconductor substrate processedaccording to the method of claim
 1. 25. A system for selectivelyirradiating structures on or within a semiconductor substrate using aplurality of laser beams, the structures being arranged in a rowextending in a generally lengthwise direction, the system comprising: ameans for generating a first laser beam that propagates along a firstlaser beam axis that intersects the semiconductor substrate; a means forgenerating a second laser beam that propagates along a second laser beamaxis that intersects the semiconductor substrate; a means for directingthe first and second laser beams onto distinct first and second spots onrespective distinct first and second structures in the row, the secondspot being offset from the first spot by some amount in a directionperpendicular to the lengthwise direction of the row; and a means formoving the first and second laser beam axes relative to thesemiconductor substrate along the row substantially in unison in adirection substantially parallel to the lengthwise direction of the row.26. A system for selectively irradiating structures on or within asemiconductor substrate using a plurality of laser beams, the structuresbeing arranged in a row extending in a generally lengthwise direction,the system comprising: a laser source producing at least a first laserbeam and a second laser beam; a first laser beam propagation path, alongwhich the first laser beam propagates toward the semiconductorsubstrate, the first laser beam propagation path having a first laserbeam axis that intersects the semiconductor substrate at a first spot; asecond laser beam propagation path, along which the second laser beampropagates toward the semiconductor substrate, the second laser beampropagation path having a second laser beam axis that intersects thesemiconductor substrate at a second spot, wherein the first spot and thesecond spot impinge upon distinct first and second structures in therow, and wherein the first and second spots are separated by somedistance in a direction perpendicular to the lengthwise direction of therow; and a motion stage that moves the first and second laser beam axesrelative to the semiconductor substrate substantially in unison in adirection substantially parallel to the lengthwise direction of the row.27. The system of claim 26, wherein the laser source comprises:respective first and second lasers.
 28. The system of claim 26, whereinthe laser source comprises: a laser; and a beam splitter disposed inboth the first and second laser beam propagation paths between the laserand the semiconductor substrate.
 29. The system of claim 26, furthercomprising: a first optical switch disposed in the first laser beampropagation path, the first optical switch capable of selectivelypassing or blocking the first laser beam from reaching the semiconductorsubstrate; and a second optical switch disposed in the second laser beampropagation path, the second optical switch capable of selectivelypassing or blocking the second laser beam from reaching thesemiconductor substrate.
 30. The system of claim 29, wherein the firstand second optical switches are AOMs.
 31. The system of claim 29,further comprising: a controller connected to the first and secondoptical switches, the controller setting the states of the first andsecond optical switches so as to irradiate only selected structures. 32.The system of claim 26, further comprising: a beam steering mechanismdisposed in the first laser beam propagation path, whereby the firstlocation can be adjusted.
 33. The system of claim 26, furthercomprising: a beam steering mechanism disposed in the second laser beampropagation path, whereby the second location can be adjusted.
 34. Thesystem of claim 26, further comprising: a beam combiner disposed in boththe first and the second laser beam propagation paths; and a focus lensdisposed in both the first and the second laser beam propagation pathsbetween the beam combiner and the semiconductor substrate.
 35. Thesystem of claim 26, further comprising: a first focus lens disposed inthe first laser beam propagation path; and a second focus lens disposedin the second laser beam propagation path.